[syslog-ng] exclude empty pairs from json-parser()

Hi - I'm trying to figure out how to instruct json-parser() to exclude empty (or nonexistent) json pairs from being passed to it's template. This is possible with the csv parser using null("") but not json. It seems that if i specify all my json-parser() pairs using --pair example=\"${MYMACRO}\" the desired behavior is achieved, however using --scope nv-pairs instead will log the empty values.