JSON: handling missing values in date coercion
xref https://github.com/pydata/pandas/issues/10746

```
In [1]: data = '''
   ...: [{"id": 1, "timestamp": 1036713600000},
   ...:  {"id": 2}]
   ...: '''
```

after #10776 this is ok

```
In [2]: pd.read_json(StringIO(data), orient='records', dtype={'id': 'int32', 'timestamp': 'datetime64[ms]'})
Out[2]: 
   id     timestamp
0   1  1.036714e+12
1   2           NaN

In [2]: pd.read_json(StringIO(data), orient='records', dtype={'id': 'int32', 'timestamp': 'datetime64[ms]'})
Out[10]: 
   id  timestamp
0   1 2002-11-08
1   2        NaT
```

still open

```
In [7]: pd.read_json(StringIO(data), orient='records', date_unit='ms')
Out[7]: 
   id     timestamp
0   1  1.036714e+12
1   2           NaN
```
