refactor: fix type

packages/backend/src/remote/resolve-user.ts

@@ -37,7 +37,7 @@ export async function resolveUser(username: string, host: string | null, option?
 		});
 	}
 
-	const user = await Users.findOne({ usernameLower, host }, option) as IRemoteUser;
+	const user = await Users.findOne({ usernameLower, host }, option) as IRemoteUser | null;
 
 	const acctLower = `${usernameLower}@${host}`;