isnull always returns False if input is np.ndarray with string inside
#### Code Sample, a copy-pastable example if possible

`pd.isnull(np.array([np.nan, 'asdf', np.nan]))`

#### Problem description

Documentation shows that `pd.isnull` should be able to take an `np.ndarray` and operate on it. This means that `np.nan` values in the array should return `True`. However, when a string is present, `np.nan` values return `False`.

#### Expected Output

Observed behaviour
```
In [3]: pd.isnull(np.array([np.nan, 'asdf', np.nan]))
Out[3]: array([False, False, False], dtype=bool)
```
Expected Behaviour
```
In [3]: pd.isnull(np.array([np.nan, 'asdf', np.nan]))
Out[3]: array([True, False, True], dtype=bool)
```

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.20.2
pytest: 3.1.2
pip: 9.0.1
setuptools: 36.0.1
Cython: None
numpy: 1.13.0
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.3
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
