BUG: TypeError in merge with timedelta64 column

[sinhrks]

• closes BUG: TypeError in merge with timedelta64 column #13389
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• 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''

[codecov-io]

Current coverage is 85.23% (diff: 100%)

Merging #13802 into master will decrease coverage by <.01%

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- Hits          42970      42969     -1   
- Misses         7445       7446     +1   
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