Complete Job Workflows without any children

Resolves #11118

getStatus of a workflow job that contains no children crashed because it
was expecting the list of children to be non-null. This fixes resolves
it by making sure that the list is an empty list instead of null.

pr-link: #11117
change-id: cid-bb824a767a143a89adfba852f465a6aa5dcfe9ca

job/server/src/main/java/alluxio/master/job/workflow/WorkflowTracker.java

@@ -218,6 +218,7 @@ private synchronized void stop(long jobId, Status status, String errorMessage) {
 
   private synchronized void next(long jobId) {
     WorkflowExecution workflowExecution = mWorkflows.get(jobId);
+    mChildren.putIfAbsent(jobId, new ConcurrentHashSet<>());
 
     Set<JobConfig> childJobConfigs = workflowExecution.next();
 
@@ -232,8 +233,6 @@ private synchronized void next(long jobId) {
     }
 
     mWaitingOn.put(jobId, childJobIds);
-
-    mChildren.putIfAbsent(jobId, new ConcurrentHashSet<>());
     mChildren.get(jobId).addAll(childJobIds);
 
     for (Long childJobId : childJobIds) {

job/server/src/test/java/alluxio/master/job/workflow/WorkflowTrackerTest.java

@@ -72,6 +72,16 @@ public void before() throws Exception {
     mMockJobServerContext = mock(JobServerContext.class);
   }
 
+  @Test
+  public void testEmpty() throws Exception {
+    ArrayList<JobConfig> jobs = Lists.newArrayList();
+    CompositeConfig config = new CompositeConfig(jobs, true);
+    mWorkflowTracker.run(config, 0);
+    WorkflowInfo info = mWorkflowTracker.getStatus(0, true);
+
+    assertEquals(Status.COMPLETED, info.getStatus());
+  }
+
   @Test
   public void testBasic() throws Exception {