Fix stale usage of emptyObject in cloneWithProps-test

After we run `require('mock-modules').dumpCache()`, the object exported by the `emptyObject` module will no longer be identical to previously exported objects, so tests like `expect(component.refs).toBe(emptyObject)` will fail.

Note that this behavior only manifests itself in tests, because of course we do not call `dumpCache` in production code.

We could consider storing the `emptyObject` globally to thwart the effects of `dumpCache`, but it's more idiomatic simply to re-`require` the latest version of `emptyObject`.

cc @petehunt