@Test
public void toJsonWithNullDefinitionReturnsExpected() {

    final ModifyThingDefinition underTest =
            ModifyThingDefinition.of(TestConstants.Thing.THING_ID, NULL_DEFINITION, DittoHeaders.empty());
    final JsonObject actualJson = underTest.toJson(FieldType.regularOrSpecial());
    assertThat(actualJson).isEqualTo(KNOWN_JSON_WITH_NULL_DEFINITION);
}