@Test
  void emptyString() {
    check(criteria.value.isEmpty()).matches("{value: ''}");
    check(criteria.optional.isEmpty()).matches("{optional: ''}");
    check(criteria.value.notEmpty()).matches("{$and: [{value: {$nin: ['', null]}}, {value: {$exists: true}}]}");
  }
{@code in} / {@code notIn} with single element converted to {@code eq} / {@code ne}
criteria/mongo/test/org/immutables/criteria/mongo/FindVisitorTest.java