IT-HF-PKG-release-prod-tomcat build should not fail if empty commit is detected

https://qa2.nuxeo.org/jenkins/job/Private/job/HFBuilding/job/IT-HF-PKG-release-prod-tomcat-nx910/54/console failed because it tries to update the changelog whereas there was nothing new because I was releasing version 1.0.1 of an existing hotfix (9.10-HF20)

12:17:48 + git commit -a -m 'Changelog update for 9.10-HF20'
12:17:48 On branch master
12:17:48 nothing to commit, working tree clean
12:17:48 Build step 'Execute shell' marked build as failure

This case should be handled and not lead to a build failure.