BUG: ordered categorical comparison with missing values evaluates to True
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
pd.Categorical(["1", "2", "3", None], categories=["1", "2", "3"], ordered=True) <= "2"
# => array([ True,  True, False,  True])
```

#### Problem description

Here a missing entry is being evaluated as `None <= "2" == True`. Shouldn't missing values always be evaluate to `False` in any comparison?
I think this is related to #4537

#### Expected Output

```python
import pandas as pd
pd.Categorical(["1", "2", "3", None], categories=["1", "2", "3"], ordered=True) <= "2"
# => array([ True,  True, False,  False])
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.2
pytest: 3.6.2
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.16.3
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.1.1
sphinx: 1.7.5
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.0.5
lxml.etree: 4.2.2
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.8
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
