@Test
  void nullMap() {
    // Act
    Map<String, Collection<String>> actualMap = caseInsensitiveCopyOf(null);
    // Assert result
    assertThat(actualMap).isNotNull().isEmpty();
  }
Ex. your {@code Foo} object would be null, but so would things like Number.
core/src/test/java/feign/UtilTest.java