code: @SuppressWarnings("unchecked")
      public static <L, M, R> ImmutableTriple<L, M, R> nullTriple() {
          return NULL;
      }
docs: Returns an immutable triple of nulls.
  
  @param <L> the left element of this triple. Value is {@code null}.
  @param <M> the middle element of this triple. Value is {@code null}.
  @param <R> the right element of this triple. Value is {@code null}.
  @return an immutable triple of nulls.
  @since 3.6