@Test
  void emptyToNullInputNullOutputNull() {
    // Arrange
    final String string = null;
    // Act
    final String retval = Util.emptyToNull(string);
    // Assert result
    assertThat(retval).isNull();
  }
Ex. your {@code Foo} object would be null, but so would things like Number.
core/src/test/java/feign/UtilTest.java