Unit Testing and good use of matchers
Well, what you do not actually test is: Are the login parameters passed to the LoginService correctly? In your example, you are simply assuming that the next login will fail or succeed, but you do that completely ignoring the actual parameters.

Especially for the successfull test I would use the actual expected parameters instead of anyString. For the not successfull test you could do the same, return a result with false if the login data is the expected value (otherwise it will be null).

This way you don't just test the way the LoginRestService handles the responses from the LoginService, but also that the LoginRestService gives the correct parameters to the LoginService in the first place, eliminating one possible cause of errors.