Convert Hive nulls to Exasol null

[morazow]

Situation

Hive null values are encoded as \N. We should convert them to Exasol null values during the import.

Acceptance Criteria

• Hive null values are correctly imported

[redcatbear]

PM decision: @morazow, please add as known issue in the docu. As long as noone explicitly asks for Hadoop 3.x support, we are shelving this.