Mockito: return default Optional.empty for unstubbed methods

mockito-optional-empty-returns.java

    public static final Answer DEFAULT_OPTIONAL_EMPTY_ANSWER = invocation ->
            invocation.getMethod().getReturnType() == Optional.class ? Optional.empty() : null;

    Mockito.mock(YourClassWithOptionalReturnValues.class, DEFAULT_OPTIONAL_EMPTY_ANSWER);
    
    // unfortunately this does not work since Mockito's Answers enum is not extendable (yet, issue 345)
    @Mock(answer = DEFAULT_OPTIONAL_EMPTY_ANSWER)
    private YourClassWithOptionalReturnValues mockInstance;