Add remaining test cases for `apply_nan_mode`

[samarth9008]

Function to add test cases for - https://github.com/kaizen-ai/kaizenflow/blob/423470c58122af11b12a1b9c3ea889abcad52342/helpers/hdataframe.py#L163

Few tests are already added over here - https://github.com/kaizen-ai/kaizenflow/blob/423470c58122af11b12a1b9c3ea889abcad52342/helpers/test/test_dataframe.py#L120

Analyze what tests are remaining and add more tests.

See unit test doc to follow the code style Also highly recommend to read this doc before submitting the PR

FYI @gpsaggese @sonaalKant @DanilYachmenev