caching: Make sender type optional for flush_cache.

That's the correct Django interface for this function.

Author: PIG208

zerver/apps.py

@@ -1,13 +1,13 @@
 import logging
-from typing import Any
+from typing import Any, Optional
 
 from django.apps import AppConfig
 from django.conf import settings
 from django.core.cache import cache
 from django.db.models.signals import post_migrate
 
 
-def flush_cache(sender: AppConfig, **kwargs: Any) -> None:
+def flush_cache(sender: Optional[AppConfig], **kwargs: Any) -> None:
     logging.info("Clearing memcached cache after migrations")
     cache.clear()