Wrong "Too many indexers" when value in series with MultiIndex is None
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
s = pd.Series([None, 'ad', 'bc', 'bd'], index=pd.MultiIndex.from_product([['a', 'b'], ['c', 'd']]))
s.loc['a', 'c']  # raises IndexingError: Too many indexers
s.loc(axis=0)['a', 'c']  # workaround
```
#### Problem description

Similar to #14885 but, in our case, the key is indeed present in the Multi Index (but the associated value is `None`).
Credit for the [workaround](https://github.com/pandas-dev/pandas/issues/14885#issuecomment-281373843) goes to @bluenote10
 

#### Expected Output
Nothing (i.e. `None` object)
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 4.0.0
pip: 18.1
setuptools: 39.1.0
Cython: None
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.8.1
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.4
lxml: None
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
