presence: Return last_update_id=-1 on zephyr mirror realm.

This was a bug, where in the realm.presence_disabled (synonymous to
being a zephyr mirror realm) case we would return None. We have decided
on the convention of using only integers here, and -1 representing lack
of data.

zerver/lib/presence.py

@@ -213,10 +213,10 @@ def get_presences_for_realm(
     slim_presence: bool,
     last_update_id_fetched_by_client: Optional[int],
     requesting_user_profile: UserProfile,
-) -> Tuple[Dict[str, Dict[str, Dict[str, Any]]], Optional[int]]:
+) -> Tuple[Dict[str, Dict[str, Dict[str, Any]]], int]:
     if realm.presence_disabled:
         # Return an empty dict if presence is disabled in this realm
-        return defaultdict(dict), None
+        return defaultdict(dict), -1
 
     return get_presence_dict_by_realm(
         realm,

zerver/tests/test_presence.py

@@ -490,6 +490,7 @@ class UserPresenceTests(ZulipTestCase):
         result = self.client_post("/json/users/me/presence", {"status": "idle"}, subdomain="zephyr")
         response_dict = self.assert_json_success(result)
         self.assertEqual(response_dict["presences"], {})
+        self.assertEqual(response_dict["presence_last_update_id"], -1)
 
     def test_mirror_presence(self) -> None:
         """Zephyr mirror realms find out the status of their mirror bot"""