Fix "collection.createIndex"

Fix "collection.createIndex" fail, when "relationshipAttributeNames" isn't defined

Author: slonofanya

lib/mongoHandler.js

@@ -153,7 +153,10 @@ MongoStore.prototype._createIndexesForRelationships = function(collection, relat
     partialIndex[name + ".id"] = 1;
     return partialIndex;
   }, {});
-  collection.createIndex(index);
+
+  if (Object.keys(index).length) {
+    collection.createIndex(index);
+  }
 };