Update packages/core/tests/mergeConfig.test.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: chenjiahan

packages/core/tests/mergeConfig.test.ts

@@ -518,7 +518,7 @@ describe('mergeRsbuildConfig', () => {
       },
     });
     expect(mergeRsbuildConfig(undefined, undefined, undefined)).toEqual(
-      undefined,
+      {},
     );
   });
 });