static KeyExtractor.Factory noKey() {
    return KeyExtractors.NoKey::new;
  }
Similar to <a href="https://en.wikipedia.org/wiki/Null_object_pattern">Null Object Pattern</a>
creates an extractor without any key. Throws exception on any extract method call. Useful when
entity doesn't have any key.
criteria/common/src/org/immutables/criteria/backend/KeyExtractor.java