[Bug] [zeta] Fix null pointer exception when submitting jobs (#6492)

Author: liugddx

Diff for: seatunnel-engine/seatunnel-engine-server/src/main/java/org/apache/seatunnel/engine/server/rest/RestHttpPostCommandProcessor.java

@@ -100,12 +100,7 @@ public void handle(HttpPostCommand httpPostCommand) {
     private SeaTunnelServer getSeaTunnelServer() {
         Map<String, Object> extensionServices =
                 this.textCommandService.getNode().getNodeExtension().createExtensionServices();
-        SeaTunnelServer seaTunnelServer =
-                (SeaTunnelServer) extensionServices.get(Constant.SEATUNNEL_SERVICE_NAME);
-        if (!seaTunnelServer.isMasterNode()) {
-            return null;
-        }
-        return seaTunnelServer;
+        return (SeaTunnelServer) extensionServices.get(Constant.SEATUNNEL_SERVICE_NAME);
     }
 
     private void handleSubmitJob(HttpPostCommand httpPostCommand, String uri)
@@ -137,7 +132,7 @@ private void handleSubmitJob(HttpPostCommand httpPostCommand, String uri)
                                 : null);
         JobImmutableInformation jobImmutableInformation = restJobExecutionEnvironment.build();
         Long jobId = jobImmutableInformation.getJobId();
-        if (seaTunnelServer == null) {
+        if (!seaTunnelServer.isMasterNode()) {
 
             NodeEngineUtil.sendOperationToMasterNode(
                             getNode().nodeEngine,
@@ -171,7 +166,7 @@ private void handleStopJob(HttpPostCommand httpPostCommand, String uri) {
         }
 
         SeaTunnelServer seaTunnelServer = getSeaTunnelServer();
-        if (seaTunnelServer == null) {
+        if (!seaTunnelServer.isMasterNode()) {
             if (isStopWithSavePoint) {
                 NodeEngineUtil.sendOperationToMasterNode(
                                 getNode().nodeEngine, new SavePointJobOperation(jobId))
@@ -183,7 +178,7 @@ private void handleStopJob(HttpPostCommand httpPostCommand, String uri) {
             }
 
         } else {
-            CoordinatorService coordinatorService = getSeaTunnelServer().getCoordinatorService();
+            CoordinatorService coordinatorService = seaTunnelServer.getCoordinatorService();
 
             if (isStopWithSavePoint) {
                 coordinatorService.savePoint(jobId);