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''
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13802?src=pr) is 85.23% (diff: 100%)

> Merging [#13802](https://codecov.io/gh/pydata/pandas/pull/13802?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13802   diff @@
==========================================
  Files           140        140          
  Lines         50415      50415          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42970      42969     -1   
- Misses         7445       7446     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [98c5b88...8dbfde2](https://codecov.io/gh/pydata/pandas/compare/98c5b88d6f9f7bb0afa4fbae49a045d93a1cb33f...8dbfde2204590455fc7e306ccdc405f91418d9eb?src=pr)
