[Java] JsonPrinter produces invalid JSON for empty messages

[Audrius-GR]

https://github.com/real-logic/simple-binary-encoding/blob/ad76638fed727295ed0f78add1359a80bff15704/sbe-tool/src/main/java/uk/co/real_logic/sbe/json/JsonTokenListener.java#L348

The intent of this line is to remove the ",\n" from the last field, but if there were no fields, it ends up removing the "{\n", producing just "}"