If you have a plugins dir configured for logstash, but it is empty Logstash will exit with an error. This seems kind of harsh. The issue seems to be that warn() is overridden in agent.rb to cause system exit rather than to just log warnings.
Interesting.. I would expect it to do nothing with the directory then.
Should be a minor fix.
I've sent a PR to fix this.
This has been merged. Resolving ticket.