Fixed Android Issue 1368 - Query View group by part of null key throws exception

Original Issue: https://github.com/couchbase/couchbase-lite-android/issues/1368

Author: hideki

src/main/java/com/couchbase/lite/store/ForestDBViewStore.java

@@ -799,9 +799,10 @@ private static boolean groupTogether(Object key1, Object key2, int groupLevel) {
 
         int end = Math.min(groupLevel, Math.min(key1List.size(), key2List.size()));
         for (int i = 0; i < end; ++i) {
-            if (!key1List.get(i).equals(key2List.get(i))) {
+            if (key1List.get(i) != null && !key1List.get(i).equals(key2List.get(i)))
+                return false;
+            else if (key1List.get(i) == null && key2List.get(i) != null)
                 return false;
-            }
         }
         return true;
     }