8239585: JFR: Native events should support empty payloads

[Erik Gahlin]

Hi,

Could I have a review of a fix that will make possible do native events 
without a payload.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8239585

Webrev:
http://cr.openjdk.java.net/~egahlin/8239585/

Testing:
jdk/jdk/jfr

Thanks
Erik