read_json with dtype=False infers Missing Values as None
Run against master:

```python
In [13]: pd.read_json("[null]", dtype=True)
Out[13]:
    0
0 NaN

In [14]: pd.read_json("[null]", dtype=False)
Out[14]:
      0
0  None
```

I think the second above is an issue - should probably return `np.nan` instead of `None`