code: public void assertContainsNull(Description description, List<?> actual) {
      assertNotNull(description, actual);
      if (!actual.contains(null)) {
        throw failures.failure(description, shouldContainNull(actual));
      }
    }
docs: Asserts that the given {@code List} contains {@code null}.
  
  @param description contains information about the assertion.
  @param actual      the given {@code Collection}.
  @throws AssertionError if the given {@code List} is {@code null}.
  @throws AssertionError if the given {@code List} does not contain null.