@Test
  void empty() {
    check(!repository.findAll().exists());
    check(!repository.find(string).exists());
    check(!repository.find(string.id.is("missing")).exists());
  }
Testing {@code exists} functionality, ie wherever at least one record could be found
for a given criteria.
criteria/common/test/org/immutables/criteria/typemodel/ExistsTemplate.java