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

...ccessful

@bfulton, please review.

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:in `info'
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:in `push_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:in `update_git_notes'