code: public static <K, V> KeyValue<K, V> empty(K key) {
          return new KeyValue<K, V>(key, null);
      }
docs: Returns an empty {@code KeyValue} instance with the {@code key} set. No value is present for this instance.
  
  @param key the key, must not be {@literal null}.
  @param <K>
  @param <V>
  @return the {@link KeyValue}