Avoid unnecessary work.

If the pending queue is empty, then we don't need to do all the work of
determining job assignments.

Bug: 141645789
Test: Android builds
Change-Id: I3dfad0ea128580ba7e924b332464b807eb2c403f

apex/jobscheduler/service/java/com/android/server/job/JobConcurrencyManager.java

@@ -606,6 +606,11 @@ class JobConcurrencyManager {
             Slog.d(TAG, printPendingQueueLocked());
         }
 
+        if (mService.getPendingJobQueue().size() == 0) {
+            // Nothing to do.
+            return;
+        }
+
         final PendingJobQueue pendingJobQueue = mService.getPendingJobQueue();
         final List<JobServiceContext> activeServices = mActiveServices;