fix: handle None from Redis in get_message_counter

The int response callback on hget raises TypeError when the key is
missing, so the None check in the mixin never actually executed.
Bypass the callback and handle the conversion manually.

modoboa/policyd/utils.py

@@ -7,5 +7,8 @@ from . import constants
 
 def get_message_counter(key):
     """Return current counter for given key."""
-    rclient = get_redis_connection()
-    return rclient.hget(constants.REDIS_HASHNAME, key)
+    rclient = get_redis_connection(hget_return_type=None)
+    value = rclient.hget(constants.REDIS_HASHNAME, key)
+    if value is None:
+        return None
+    return int(value)