@Test public void tag_customizer_empty() {

  when(parseValue.apply(input, null)).thenReturn("");
  tag.tag(input, customizer);
  verify(parseValue).apply(input, null);
  verifyNoMoreInteractions(parseValue); // doesn't parse twice
  verify(customizer).tag("key", "");
  verifyNoMoreInteractions(customizer); // doesn't tag twice
}