Ensure checking task status doesn't unecessarily raise an exception (s…

…potify#52)

Based of off Netflix@b8d1b95 as we saw the same issue internally

lemur/common/celery.py

@@ -79,6 +79,10 @@ def is_task_active(fun, task_id, args):
 
     i = inspect()
     active_tasks = i.active()
+
+    if active_tasks is None:
+        return False
+
     for _, tasks in active_tasks.items():
         for task in tasks:
             if task.get("id") == task_id: