Add NullListener to suppress nil exceptions when listener lookup is unsu...

[peakxu]

...ccessful

@bfulton, please review.

Not sure where CI for this is meant to run. Travis? Ran specs locally and they passed.

Motivation: a recent build failure https://jenkins.swipely.com/view/app/job/app.service_performance_pipeline/lastFailedBuild/console

Relevant stack trace:
12:22:32.645 git-notes plugin: returning results of run: {:out=>"commit dcdf54d62b1aa1a56403e121f35f7a1da347b014\nMerge: 61f5d44 2d5d0c3\nAuthor: Peak Xu peak.xu@gmail.com\nDate: Mon Oct 6 12:20:13 2014 -0400\n\n Merge pull request #33 from dotswipely/deterministic_server_name\n \n Merge strings in aggregating hash in deterministic order\n\n", :err=>"", :val=>0}
12:22:33.398 ERROR: undefined method info' for nil:NilClass (NoMethodError) 12:22:33.399 /var/lib/jenkins/plugins/git-notes/WEB-INF/classes/lib/build_participant.rb:29:ininfo'
12:22:33.399 /var/lib/jenkins/plugins/git-notes/WEB-INF/classes/lib/build_participant.rb:58:in run' 12:22:33.399 /var/lib/jenkins/plugins/git-notes/WEB-INF/classes/lib/git_updater.rb:37:inpush_notes'
12:22:33.399 /var/lib/jenkins/plugins/git-notes/WEB-INF/classes/lib/git_updater.rb:14:in update!' 12:22:33.399 /var/lib/jenkins/plugins/git-notes/WEB-INF/classes/models/git_notes_publisher.rb:50:inupdate_git_notes'

[bfulton]

It's disturbing that the listener is sometimes not available. We need to be able to rely on our logs for debugging, so I never want to silently not log. How about falling back to puts?

[peakxu]

Very good point. Now prints to stderr.

BTW, agreed on this being disturbing. Curious about what the underlying issue is. This PR is more of a quick bandaid.