Comparing Timedelta and NaT gives inconsistent results depending on order
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd                                                                                                                                                 

In [2]: pd.Timedelta(0) > pd.Timedelta(None)                                              
TypeError: Cannot compare type Timedelta with type NaTType

In [3]: pd.Timedelta(None) < pd.Timedelta(0)                                                                                                                                            
Out[3]: False

```
#### Problem description

I would expect the result of `a < b` to be the same as that of `b > a` but in this case one returns `False` and the other raises `TypeError`.

Related issue: #24983 "Separate NaT values for Timedelta and Period" - if `pd.Timedelta(None)` gave a value of type `Timedelta` then maybe this error would not have occurred.

#### Expected Output

Either both `False` or both `TypeError` (both `False` would be more helpful)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.2
pytest: 4.3.1
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.5
numpy: 1.16.2
scipy: None
pyarrow: None
xarray: 0.11.3
IPython: 7.3.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.18
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
