public Object resolve(final Expression expression) {
    return new Visitor().process(expression, null);
  }
@Test
  public void shouldResolveNullLiteral() {
    // Given:
    final SqlType type = SqlTypes.STRING;
    final Expression exp = new NullLiteral();

    // When:
    final Object o = new GenericExpressionResolver(type, FIELD_NAME, registry, config,
        "insert value", false).resolve(exp);

    // Then:
    assertThat(o, Matchers.nullValue());
  }