BUG: preserve dtype for right/outer merge of datetime with different resolutions
Follow-up on https://github.com/pandas-dev/pandas/pull/53213, fixing one more corner case for the right/outer joins making the key column object dtype.

Expanded the test added in that PR to test the different join types. 
The whatsnew added in https://github.com/pandas-dev/pandas/pull/53213 should also already cover this.
Thanks @jorisvandenbossche 