you should use required/requiredMessages attribute. And this is not RF related problem but standard JSF behavior.
Yes, I agree that this is JSF behaviour, but using "required" attribute causes default not custom validators to be invoked.
I don't like the message given by default validator and that's why I want to use custom one, but it doesn't work, if the field is empty.
I wonder, if there is a work around, which would allow me to attach custom message.