IMPALA-925: JDBC driver returns no results from getTables()/Columns() with null name pattern

Our HS2 Metadata Op implementation would not return any results if null was passed as the
table name or column name. Instead a null value should be treated the same as '%' (match
everything).

Change-Id: Ibad41e94724cd1f9c1caf40831e30a98132247d9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2137
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins