DataFrame sort_values and multiple "by" columns fails to order NaT correctly
* Removed unnecessary conversion to i8
* Fixed failed test (`test_frame_column_inplace_sort_exception`)
* Added check to ensure that the test is performing its intended goal(`test_sort_nan`)

 - [x] closes #16836 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry
