Do not allow a None-limit on PaginationConfig. (#14146)

The callers either set a default limit or manually handle a None-limit
later on (by setting a default value).

Update the callers to always instantiate PaginationConfig with a default
limit and then assume the limit is non-None.

tests/rest/client/test_typing.py

@@ -59,7 +59,8 @@ class RoomTypingTestCase(unittest.HomeserverTestCase):
             self.event_source.get_new_events(
                 user=UserID.from_string(self.user_id),
                 from_key=0,
-                limit=None,
+                # Limit is unused.
+                limit=0,
                 room_ids=[self.room_id],
                 is_guest=False,
             )