BUG: "IndexingError: Too many indexers" when accessing a None value using .loc through a MultiIndex
#### Steps to reproduce

```python
print(pd.Series(
    [None],
    pd.MultiIndex.from_arrays([['Level1'], ['Level2']]))
    .loc[('Level1', 'Level2')])
```

#### Expected output

```
None
```

#### Actual output

```
/usr/local/lib/python3.6/dist-packages/pandas/core/indexing.py in __getitem__(self, key)
   1760                 except (KeyError, IndexError, AttributeError):
   1761                     pass
-> 1762             return self._getitem_tuple(key)
   1763         else:
   1764             # we by definition only have the 0th axis

/usr/local/lib/python3.6/dist-packages/pandas/core/indexing.py in _getitem_tuple(self, tup)
   1275 
   1276         # no multi-index, so validate all of the indexers
-> 1277         self._has_valid_tuple(tup)
   1278 
   1279         # ugly hack for GH #836

/usr/local/lib/python3.6/dist-packages/pandas/core/indexing.py in _has_valid_tuple(self, key)
    699         for i, k in enumerate(key):
    700             if i >= self.ndim:
--> 701                 raise IndexingError("Too many indexers")
    702             try:
    703                 self._validate_key(k, i)

IndexingError: Too many indexers
```

#### Additional information

If any value other than `None` (even `np.nan`) is used, the code behaves correctly.

If a single index level is used, the code behaves correctly.

#### Workaround

Seems to work if `.loc(axis=0)[('Level1', 'Level2')]` is used instead.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.3.candidate.1
python-bits      : 64
OS               : Linux
OS-release       : 5.6.0-1-amd64
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3
Cython           : None
pytest           : 4.6.9
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 4.6.9
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
