Fix JSON builder when missing keys in first row 
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<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.009262 / 0.011353 (-0.002091) | 0.006157 / 0.011008 (-0.004851) | 0.125960 / 0.038508 (0.087451) | 0.036213 / 0.023109 (0.013104) | 0.399331 / 0.275898 (0.123433) | 0.453597 / 0.323480 (0.130117) | 0.006990 / 0.007986 (-0.000995) | 0.007320 / 0.004328 (0.002991) | 0.100321 / 0.004250 (0.096070) | 0.048870 / 0.037052 (0.011818) | 0.396284 / 0.258489 (0.137795) | 0.475619 / 0.293841 (0.181778) | 0.052329 / 0.128546 (-0.076217) | 0.019564 / 0.075646 (-0.056083) | 0.430942 / 0.419271 (0.011670) | 0.063224 / 0.043533 (0.019692) | 0.391717 / 0.255139 (0.136578) | 0.448342 / 0.283200 (0.165142) | 0.114055 / 0.141683 (-0.027628) | 1.793204 / 1.452155 (0.341049) | 1.895151 / 1.492716 (0.402435) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.283699 / 0.018006 (0.265693) | 0.597194 / 0.000490 (0.596704) | 0.007143 / 0.000200 (0.006944) | 0.000602 / 0.000054 (0.000548) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.034761 / 0.037411 (-0.002651) | 0.124555 / 0.014526 (0.110030) | 0.149126 / 0.176557 (-0.027430) | 0.220335 / 0.737135 (-0.516801) | 0.153109 / 0.296338 (-0.143229) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.620210 / 0.215209 (0.405001) | 6.229937 / 2.077655 (4.152282) | 2.615203 / 1.504120 (1.111083) | 2.239337 / 1.541195 (0.698143) | 2.262138 / 1.468490 (0.793648) | 1.196498 / 4.584777 (-3.388279) | 5.609932 / 3.745712 (1.864220) | 3.031347 / 5.269862 (-2.238515) | 2.025530 / 4.565676 (-2.540146) | 0.139828 / 0.424275 (-0.284447) | 0.015476 / 0.007607 (0.007869) | 0.768964 / 0.226044 (0.542920) | 7.728677 / 2.268929 (5.459748) | 3.336407 / 55.444624 (-52.108217) | 2.700055 / 6.876477 (-4.176422) | 2.765223 / 2.142072 (0.623151) | 1.409073 / 4.805227 (-3.396155) | 0.246849 / 6.500664 (-6.253815) | 0.081231 / 0.075469 (0.005762) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.593836 / 1.841788 (-0.247952) | 18.020525 / 8.074308 (9.946216) | 21.766822 / 10.191392 (11.575430) | 0.258615 / 0.680424 (-0.421809) | 0.026895 / 0.534201 (-0.507306) | 0.529823 / 0.579283 (-0.049460) | 0.623470 / 0.434364 (0.189106) | 0.628171 / 0.540337 (0.087833) | 0.745249 / 1.386936 (-0.641687) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.008624 / 0.011353 (-0.002729) | 0.006317 / 0.011008 (-0.004691) | 0.097315 / 0.038508 (0.058807) | 0.035217 / 0.023109 (0.012108) | 0.440197 / 0.275898 (0.164299) | 0.473863 / 0.323480 (0.150383) | 0.006722 / 0.007986 (-0.001264) | 0.006444 / 0.004328 (0.002116) | 0.102056 / 0.004250 (0.097806) | 0.047142 / 0.037052 (0.010089) | 0.452476 / 0.258489 (0.193986) | 0.487619 / 0.293841 (0.193778) | 0.052456 / 0.128546 (-0.076090) | 0.018735 / 0.075646 (-0.056911) | 0.114656 / 0.419271 (-0.304616) | 0.062577 / 0.043533 (0.019044) | 0.444471 / 0.255139 (0.189332) | 0.494264 / 0.283200 (0.211065) | 0.117112 / 0.141683 (-0.024571) | 1.848965 / 1.452155 (0.396810) | 1.984008 / 1.492716 (0.491292) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.290494 / 0.018006 (0.272488) | 0.588415 / 0.000490 (0.587925) | 0.000459 / 0.000200 (0.000259) | 0.000080 / 0.000054 (0.000026) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.032873 / 0.037411 (-0.004538) | 0.131139 / 0.014526 (0.116614) | 0.140268 / 0.176557 (-0.036289) | 0.204561 / 0.737135 (-0.532574) | 0.147443 / 0.296338 (-0.148895) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.636899 / 0.215209 (0.421690) | 6.236139 / 2.077655 (4.158484) | 2.801468 / 1.504120 (1.297348) | 2.398808 / 1.541195 (0.857613) | 2.493150 / 1.468490 (1.024659) | 1.228845 / 4.584777 (-3.355932) | 5.675874 / 3.745712 (1.930162) | 3.084939 / 5.269862 (-2.184922) | 2.061310 / 4.565676 (-2.504367) | 0.142285 / 0.424275 (-0.281990) | 0.014972 / 0.007607 (0.007365) | 0.786599 / 0.226044 (0.560555) | 7.876036 / 2.268929 (5.607107) | 3.476136 / 55.444624 (-51.968489) | 2.847922 / 6.876477 (-4.028555) | 3.040326 / 2.142072 (0.898253) | 1.448538 / 4.805227 (-3.356690) | 0.257230 / 6.500664 (-6.243434) | 0.085137 / 0.075469 (0.009668) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.668173 / 1.841788 (-0.173615) | 18.668520 / 8.074308 (10.594212) | 20.535542 / 10.191392 (10.344150) | 0.244580 / 0.680424 (-0.435844) | 0.026364 / 0.534201 (-0.507837) | 0.531753 / 0.579283 (-0.047530) | 0.616578 / 0.434364 (0.182214) | 0.618906 / 0.540337 (0.078569) | 0.738785 / 1.386936 (-0.648151) |

</details>
</details>

![](https://cml.dev/watermark.png#f7265cafa3103d77d6d52aa897088faefcd96659 "CML watermark")

Until now, the JSON builder only considered the keys present in the first element of the list:
- Either explicitly: by passing index 0 in `dataset[0].keys()`
- Or implicitly: `pa.Table.from_pylist(dataset)`, where "schema (default None): If not passed, will be inferred from the first row of the mapping values"

This PR fixes the bug by considering the union of the keys present in all the rows.


Fix #5726.