If target table are empty, the following query
will throw warning
This is caused by an HDFS bug https://issues.apache.org/jira/browse/HDFS-8407
hdfsListDirectory() returns NULL when the directory is empty.
According to the API doc, it should only return NULL when there is an error.
In pre 2.2 version, we don't hit this HDFS bug because of
IMPALA-1438 so the directory will not be empty.