BUG: DataFrame.min/max with axis=1 and uniform datetime64[ns, tz] types does not return NaNs
- [x] closes #10390
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Essentially, check that all blocks are are datetimetz type and the same timezone before performing `max`/`min` since datetimetz blocks are only 1D