Only create traverserEngines that actually have shards

arangod/Aql/ExecutionEngine.cpp

@@ -920,6 +920,7 @@ struct CoordinatorInstanciator : public WalkerWorker<ExecutionNode> {
 
       engineInfo.close(); // base
 
+      if (!shardSet.empty()) {
         arangodb::CoordTransactionID coordTransactionID = TRI_NewTickServer();
         std::unordered_map<std::string, std::string> headers;
 
@@ -952,6 +953,7 @@ struct CoordinatorInstanciator : public WalkerWorker<ExecutionNode> {
         }
       }
     }
+  }
 
   /// @brief buildEngines, build engines on DBservers and coordinator
   ExecutionEngine* buildEngines() {