Worker: properly handle task updates without changing task status.

packages/flamenco-worker-python/flamenco_worker/worker.py

@@ -223,6 +223,9 @@ class FlamencoWorker:
         for key, value in kwargs.items():
             setattr(self.last_task_activity, key, value)
 
+        if task_status is None:
+            task_status_changed = False
+        else:
             task_status_changed = self.current_task_status != task_status
             self.current_task_status = task_status