Handle empty dataframe in OrgaosMappingCleaner

[augusto-herrmann]

If OrgaosMappingCleaner receives an empty dataframe (e.g. when doing an incremental database load and there is no change in data), it fails with the error

KeyError: "None of [Index(['nan'], dtype='object', name='key_name')] are in the [index]"

It should instead handle gracefully, log the occurrence of an empty dataframe and return without error so that the Airflow task can succeed.