Don't throw when commitId is null

Last week, we added the Flowdock-Jenkins plugin to our Jenkins installation. The first time the project ran, the build showed as failed. The reason was because the plugin threw a null reference exception because the commitId was null. Honestly, I don't know why it was null and didn't care much; I just didn't want it to throw. I compiled this new code, added it to our Jenkins, and everything worked great. I'd like it to be brought into the main project so I can continue to use your official plugin in Jenkins.