Fix server crash when trying to execute activate_node_snapshot() on a single-node cluster (#7552)

This fixes #7551 reported by Egor Chindyaskin

Function activate_node_snapshot() is not meant to be called on a cluster
without worker nodes. This commit adds ERROR report for such case to
prevent server crash.

src/backend/distributed/test/metadata_sync.c

@@ -50,6 +50,13 @@ activate_node_snapshot(PG_FUNCTION_ARGS)
 	 * so we are using first primary worker node just for test purposes.
 	 */
 	WorkerNode *dummyWorkerNode = GetFirstPrimaryWorkerNode();
+	if (dummyWorkerNode == NULL)
+	{
+		ereport(ERROR, (errmsg("no worker nodes found"),
+						errdetail("Function activate_node_snapshot is meant to be "
+								  "used when running tests on a multi-node cluster "
+								  "with workers.")));
+	}
 
 	/*
 	 * Create MetadataSyncContext which is used throughout nodes' activation.