BUG: Type mismatch in read_json

Tested on Pandas v1.1.0

I'm trying to import a JSON file containing the following:

**test.json**
````json
{"file_path": null}
````

If I run the following code,

````python
pd.read_json("test.json", typ='series').to_dict()
````

I get this output:
````python
{'file_path': NaT}
````

Why is `null` getting converted to `NaT` instead of `None`?

