Asymmetric behavior between index and columns when getting incomplete label
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame([[1,2], [3,4]], index=pd.MultiIndex.from_tuples([['a', 'b'], ['c', '']]))

In [3]: df.loc['c'].shape
Out[3]: (1, 2)

In [4]: df.transpose().loc[:, 'c'].shape
Out[4]: (2,)
```
#### Problem description

Maybe the "fill an incomplete key with empty string(s)" rule is not implemented at all for rows? (also in light of #17024 ) If this the case, then I think it should be.

#### Expected Output

The same as ``Out[4]`` but reversed.

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

<details>
INSTALLED VERSIONS
------------------
commit: 9e7666dae3b3b10d987ce154a51c78bcee6e0728
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-3-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.21.0.dev+265.g9e7666dae
pytest: 3.0.6
pip: 9.0.1
setuptools: None
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.1.0.dev
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.2
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.2.1
</details>
