read_json Raises AttributeError with Valid JSON as Input
#### A small, complete example of the issue

``` python
pd.read_json('[{}, null]')  # This raises `AttributeError`
pd.read_json('[null, {}]')  # This does not
```

While this isn't a big problem, the output is not as expected. The only way I see this causing issues for me is for parsing JSON received on a server; the raised error does not signify bad JSON (which it isn't) nor an appropriate ValueError. I don't think this should raise an error at all actually, correct me if I'm wrong.
#### Expected Output

``` python
# Raises `AttributeError`
      0
0    {}
1  None
# This is as expected
      0
0  None
1    {}
```
#### Actual Output

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/pandas/io/json.py", line 211, in read_json
    date_unit).parse()
  File "/usr/local/lib/python2.7/dist-packages/pandas/io/json.py", line 279, in parse
    self._parse_no_numpy()
  File "/usr/local/lib/python2.7/dist-packages/pandas/io/json.py", line 496, in _parse_no_numpy
    loads(json, precise_float=self.precise_float), dtype=None)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 263, in __init__
    arrays, columns = _to_arrays(data, columns, dtype=dtype)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 5355, in _to_arrays
    coerce_float=coerce_float, dtype=dtype)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 5471, in _list_of_dict_to_arrays
    columns = lib.fast_unique_multiple_list_gen(gen)
  File "pandas/lib.pyx", line 504, in pandas.lib.fast_unique_multiple_list_gen (pandas/lib.c:10190)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 5470, in <genexpr>
    gen = (list(x.keys()) for x in data)
AttributeError: 'NoneType' object has no attribute 'keys'
```
#### Output of `pd.show_versions()`

<details>
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-38-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 25.1.1
Cython: None
numpy: 1.11.1
scipy: 0.18.0
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
