Fix up shutdown for periodic executors

Author: Andrey Gavrilin

pymongo/periodic_executor.py

@@ -126,6 +126,9 @@ def _on_executor_deleted(ref):
 
 
 def _shutdown_executors():
+ if _EXECUTORS is None:
+ return
+
  # Copy the set. Stopping threads has the side effect of removing executors.
  executors = list(_EXECUTORS)