fix(mocker): don't restore mock to the original if module is mocked

packages/vitest/src/runtime/mocker.ts

@@ -278,7 +278,7 @@ export class VitestMocker {
           const mock = spyOn(newContainer, property).mockImplementation(() => undefined)
           mock.mockRestore = () => {
             mock.mockReset()
-            mock.mockImplementation(undefined!)
+            mock.mockImplementation(() => undefined)
             return mock
           }
           // tinyspy retains length, but jest doesn't.