pass temp directory only when present
When the temp_directory parameter was not passed to HiveToDruidOperator(), the code was still passing the parameter --temp_directory=None to HiveToDruid.scala. This was making the scala job fail. The parameter should only be passed to HiveToDruid.scala if it is present. This change fixes it.
NOTE: this was not caught in the tests, since I was always setting the temp_directory parameter to write in a non-production space.