code: @SuppressWarnings("unchecked")
      public static <M extends PMessage<M,F>, F extends PField, B extends PMessageBuilder<M,F>>
      B mutateIfNotNull(M message) {
          if (message == null) {
              return null;
          }
          return (B) message.mutate();
      }
docs: Mutate the message if it is not null.
  
  @param message Message to mutate.
  @param <M> The message type.
  @param <F> The field type.
  @param <B> The builder type.
  @return The builder or null if null input.