Fix OpenTSDB#794 by making sure the group bys and row key literals li…

…st is

not null before checking if it's empty.

Signed-off-by: Chris Larsen <clarsen@yahoo-inc.com>

src/query/QueryUtil.java

@@ -200,7 +200,8 @@ public static void setDataTableScanFilter(
       final int end_time) {
 
     // no-op
-    if (group_bys.isEmpty() && row_key_literals.isEmpty()) {
+    if ((group_bys == null || group_bys.isEmpty()) 
+        && (row_key_literals == null || row_key_literals.isEmpty())) {
       return;
     }