TAJO-842: NULL handling in JDBC. (Hyoungjun Kim via jinho)

Closes #18
TAJO-842: NULL handling in JDBC. (fixed a missing log in CHANGES)
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-840
Adding a null check condition in DistinctGroupbyHashAggregationExec.