Formatting of CategoricalIndex misaligned if NaNs are present
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> pd.DataFrame([[1],[2]], index=pd.CategoricalIndex([1, pd.np.nan]))
      0
 1.0  1
NaN   2
```
#### Problem description

`CategoricalIndex` of a `DataFrame` is misaligned when printing if it contains `NaN` values.

#### Expected Output

```python
>>> import pandas as pd
>>> pd.DataFrame([[1],[2]], index=pd.CategoricalIndex([1, pd.np.nan]))
      0
1.0   1
NaN   2
```

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

<details>

No module named 'dask'

INSTALLED VERSIONS
------------------
commit: da5f5eb6c11140a2bb9f84049d30ba7d89bdbfd0
python: 3.6.6.final.0
python-bits: 64
OS: Darwin
OS-release: 18.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.24.0.dev0+1684.gda5f5eb6c
pytest: 4.0.0
pip: 10.0.1
setuptools: 40.6.2
Cython: 0.29
numpy: 1.14.2
scipy: 1.1.0
pyarrow: 0.11.1
xarray: 0.11.0
IPython: 7.1.1
sphinx: 1.8.2
patsy: 0.5.0
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.9
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.1.2
lxml.etree: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.14
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: 0.1.6
fastparquet: 0.2.1
pandas_gbq: None
pandas_datareader: None
gcsfs: 0.1.2

</details>
