Fix _reset_conn_state to handle None db_config

When finalizer() sets connections._db_config = None and another test
runs on the same worker (with pytest-xdist), _reset_conn_state() would
fail because it tried to call .clear() on None.

This fix checks if _db_config is not None before clearing it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: abondar

tortoise/contrib/test/__init__.py

@@ -271,7 +271,8 @@ async def asyncSetUp(self) -> None:
     def _reset_conn_state(self) -> None:
         # clearing the storage and db config
         connections._clear_storage()
-        connections.db_config.clear()
+        if connections._db_config is not None:
+            connections._db_config.clear()
 
     async def asyncTearDown(self) -> None:
         await self._tearDownDB()