public static ValueReference ofNullable(@Nullable String value) {
        if (value == null) {
            return null;
        } else {
            return of(value);
        }
    }
@Test
    public void testOfNullable() {
        assertThat(ValueReference.ofNullable("test")).isNotNull();
        assertThat(ValueReference.ofNullable((String) null)).isNull();
        assertThat(ValueReference.ofNullable(TestEnum.A)).isNotNull();
        assertThat(ValueReference.ofNullable((TestEnum) null)).isNull();
    }