Test to datetime null to NaT
- [x] closes #35888
- [x] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

Looks like there's typing errors:

```
pandas/tests/tools/test_to_datetime.py:1074: error: List item 0 has incompatible type "None"; expected "NaTType"  [list-item]
``` @mroeschke - thanks for the review. I'll reduce the size of the test series. I've now got the dev environment set up to run mypy locally now. But I'm unsure how to change the code to avoid typing errors.

Mypy complains with either of the following `Series` objects because they have an incompatible type. 

```
Series([NaT] * 20 + [None] * 20, dtype="object")
Series([None] * 20 + [NaT] * 20, dtype="object")
```

Possible fixes: 

- use `# type: ignore` 
   - not seen this in any other tests, so doesn't seem appropriate 
- ignore the test case 
   - is it not a valid test case? 
   - it is a reasonable/possible input to the `to_datetime()` function, so I feel it ought to be tested 
 - something else? I would just use `# type: ignore[list-item]` for now, and if you're interested you can look into adjusting the typing of `to_datetime` in a follow up PR. I think we DO need to test a larger input (as well) here e.g. 'It only occurs if the input is large enough to force the caching mechanism.' @ryangilmour can you merge master and address comments Yep - i'll get round to this in the next few days.  thanks @ryangilmour 