Fix+Test Timedelta.__mul__(nan)
Specifically, `np.float64("NAN")` which in master gets caught by the `hasattr(other, "dtype")` branch and incorrectly returns `np.timedelta64("NaT", "ns")` instead of `NaT`

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_arithmetic.py`](https://github.com/jbrockmendel/pandas/blob/6588e199b1204923c930d37567dea35e68827a8d/pandas/tests/scalar/timedelta/test_arithmetic.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23342?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@c76173b`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23342/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23342?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23342   +/-   ##
=========================================
  Coverage          ?   92.22%           
=========================================
  Files             ?      169           
  Lines             ?    51217           
  Branches          ?        0           
=========================================
  Hits              ?    47236           
  Misses            ?     3981           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <ø> (?)` | |
| #single | `42.23% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23342?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23342?src=pr&el=footer). Last update [c76173b...6588e19](https://codecov.io/gh/pandas-dev/pandas/pull/23342?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks