fix(watcher): getDifference needs a date, and only a completed drain counts

backend/src/watcher/registry.py

@@ -185,6 +185,8 @@ class BotRegistry:
 
     async def _backlog(self, client: PyroClient, bot_id: int, account_id: int) -> None:
         stored = await drain_backlog(client, account_id)
+        if stored is None:
+            return
         await bots.mark_backfilled(self._pool, bot_id)
         if stored:
             await accounts.notify_accounts_changed(self._pool)