-fix: DELETE FROM index:<indexName> WHERE key = <key>, were failing o…

…n composite key, when rid wasn't specified.

       expected behavior, to remove all values under specified (partial/full) key
-remarks: all test passed after fix

core/src/main/java/com/orientechnologies/orient/core/index/OIndexMultiValues.java

@@ -210,7 +210,10 @@ public boolean remove(Object key, final OIdentifiable value) {
             return false;
           }
 
-          if (values.remove(value)) {
+          if (value == null) {
+            indexEngine.remove(key);
+          }
+          else if (values.remove(value)) {
             if (values.isEmpty())
               indexEngine.remove(key);
             else