[Improve][RestAPI] always return jobId when call getJobInfoById API (#6422)

Author: liunaijie

Diff for: docs/en/seatunnel-engine/rest-api.md

@@ -111,6 +111,14 @@ network:
 }
 ```
 
+When we can't get the job info, the response will be:
+
+```json
+{
+  "jobId" : ""
+}
+```
+
 </details>
 
 ------------------------------------------------------------------------------------------

Diff for: seatunnel-e2e/seatunnel-engine-e2e/connector-seatunnel-e2e-base/src/test/java/org/apache/seatunnel/engine/e2e/RestApiIT.java

@@ -108,6 +108,40 @@ public void testGetRunningJobById() {
                         });
     }
 
+    @Test
+    public void testGetAnNotExistJobById() {
+        Arrays.asList(node2, node1)
+                .forEach(
+                        instance -> {
+                            given().get(
+                                            HOST
+                                                    + instance.getCluster()
+                                                            .getLocalMember()
+                                                            .getAddress()
+                                                            .getPort()
+                                                    + RestConstant.RUNNING_JOB_URL
+                                                    + "/"
+                                                    + 123)
+                                    .then()
+                                    .statusCode(200)
+                                    .body("jobId", equalTo("123"));
+                        });
+        Arrays.asList(node2, node1)
+                .forEach(
+                        instance -> {
+                            given().get(
+                                            HOST
+                                                    + instance.getCluster()
+                                                            .getLocalMember()
+                                                            .getAddress()
+                                                            .getPort()
+                                                    + RestConstant.RUNNING_JOB_URL
+                                                    + "/")
+                                    .then()
+                                    .statusCode(500);
+                        });
+    }
+
     @Test
     public void testGetRunningJobs() {
         Arrays.asList(node2, node1)

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

@@ -275,7 +275,7 @@ private void handleJobInfoById(HttpGetCommand command, String uri) {
         if (!jobId.isEmpty() && jobInfo != null) {
             this.prepareResponse(command, convertToJson(jobInfo, Long.parseLong(jobId)));
         } else {
-            this.prepareResponse(command, new JsonObject());
+            this.prepareResponse(command, new JsonObject().add(RestConstant.JOB_ID, jobId));
         }
     }