[syslog5424] confirm process ID is nil when RFC NILVALUE used
[syslog5424] structured data should be nil when RFC NILVALUE is used
[syslog5424] non-capturing parens since no backreferences needed
Merge pull request #586 from bfritz/syslog5424-fixes

nil instead of "-" for missing structured data with grok pattern for rfc 5424 syslog