Fix NPE

Author: ivancea

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plugin/DataNodeComputeHandler.java

@@ -109,7 +109,7 @@ void startComputeOnDataNodes(
             esqlExecutor,
             parentTask,
             configuration.allowPartialResults(),
-            maxConcurrentNodesPerCluster
+            maxConcurrentNodesPerCluster == null ? -1 : maxConcurrentNodesPerCluster
         ) {
             @Override
             protected void sendRequest(