IMPALA-883: COMPUTE STATS returns -1 for number of rows in empty partition.

The query used to generate the stats does a GROUP BY on the partition keys,
and so empty partitions will not get any results.  Detect the empty partition
case and set the number of rows to 0.

Change-Id: I1ccb7d2016f35026aa1b418155c4534024f3cee5
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4029
Reviewed-by: Daniel Hecht <dhecht@cloudera.com>
Tested-by: jenkins