Fix TypeError in update_lane_size script (#1420)

Add default of None to _db arg for the update_lane_size script.

Fixes this traceback:
  File "bin/update_lane_size", line 11, in <module>
    UpdateLaneSizeScript().run()
TypeError: __init__() missing 1 required positional argument: '_db'

core/scripts.py

@@ -2645,7 +2645,7 @@ def add_line(id, name, protocol, metadata_identifier):
 
 
 class UpdateLaneSizeScript(LaneSweeperScript):
-    def __init__(self, _db, *args, **kwargs):
+    def __init__(self, _db=None, *args, **kwargs):
         super().__init__(_db, *args, **kwargs)
         search = kwargs.get("search_index_client", None)
         self._search: ExternalSearchIndex = search or ExternalSearchIndex(self._db)