Fix crash in NotificationFlag.flag_names when flags is None

flag_names() crashes with AttributeError when NotificationFlag returns
a value whose .name attribute is None. This occurs with certain receiver
firmware versions (Nano C52F, Unifying C52B, Bolt C548).

Return an empty list instead, consistent with the List[str] return type.

Fixes #3184

lib/logitech_receiver/hidpp10_constants.py

@@ -91,6 +91,8 @@ class NotificationFlag(IntFlag):
     @classmethod
     def flag_names(cls, flags) -> List[str]:
         """Extract the names of the flags from the integer."""
+        if flags is None or flags.name is None:
+            return []
         return flags.name.replace("_", " ").lower().split("|")
 
     NUMPAD_NUMERICAL_KEYS = 0x800000