fix: Impala strings/objects now return None instead of NaN (#406)
fix: Impala strings/objects now return None instead of NaN (#406)

* fix: strings/objects now return None instead of NaN

* Previously Impala strings, chars and other objects filled a dataframe with NaN
instead of NULL which caused mismatch issues when comparing with BigQuery dataframes.
* Moved _column_batches_to_dataframe and _chunks_to_pandas_array functions from the ibis.backends.impala.client into api.py to override default functionality.
* refactor: import instead of rewrite _HS2_TTypeId_to_dtype
GoogleCloudPlatform/professional-services-data-validator,GoogleCloudPlatform/professional-services-data-validator