BUG: TypeError in merge with timedelta64 column
- [x] closes #13389
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

A root cause is `isnull` raises `TypeError` if datetime-like has other time unit than `ns`.

```
pd.isnull(np.array([1, 2], dtype='datetime64[D]'))
# TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''
```
