Map function: Type error on iter #999
## Describe the bug
Using the map function, it throws a type error on iter #999

Here is the code I am calling:
```
dataset = datasets.load_dataset('squad')
dataset['validation'].map(text_numbers_to_int, input_columns=['context'], fn_kwargs={'column': 'context'})
``` 
text_numbers_to_int returns the input text with numbers replaced in the format {'context': text}

It happens at 
`
File "C:\Users\lonek\anaconda3\envs\ai\Lib\site-packages\datasets\arrow_writer.py", line 289, in <listcomp>
    [row[0][col] for row in self.current_examples], type=col_type, try_type=col_try_type, col=col
`

The issue is that the list comprehension expects self.current_examples to be type tuple(dict, str), but for some reason 26 out of 1000 of the sefl.current_examples are type tuple(str, str)

Here is an example of what self.current_examples should be
({'context': 'Super Bowl 50 was an...merals 50.'}, '')

Here is an example of what self.current_examples are when it throws the error:
('The Panthers used th... Marriott.', '')
