How to test a method returns boolean in Mockito

I am learning how to unit-testing in android studio. As shown below, I have a method called "isValidUrl" and in the testing section below, I coded the testing of that method using Mockito, but the test always fails.

Can you please help and guild me how to test this method?

code

public boolean isValidUrl(String url) {
    return (url != null && !url.equals("")) ? true : false;
}

testing:

public class ValidationTest {
@Mock
private Context mCtx = null;

@Before
public void setUp() throws Exception {
    mCtx = Mockito.mock(Context.class);
    Assert.assertNotNull("Context is not null", mCtx);
}

@Test
public void isValidUrl() throws Exception {
    Validation validation = new Validation(mCtx);
    String url = null;
    Mockito.when(validation.isValidUrl(url)).thenReturn(false);
}

}