code: public static <M extends PMessage<M,F>, F extends PField, B extends PMessageBuilder<M,F>>
      M buildIfNotNull(B builder) {
          if (builder == null) {
              return null;
          }
          return builder.build();
      }
docs: Build the message from builder if it is not null.
  
  @param builder The builder to build.
  @param <M> The message type.
  @param <F> The field type.
  @param <B> The builder type.
  @return The message or null if null input.