Only track project credentials if last build exists
Only track project credentials if last build exists

[JENKINS-44087] detected a null pointer exception in the git parameters
plugin when it was used before the first run of a job.

martinda/git-plugin,recena/git-plugin,v1v/git-plugin,jacob-keller/git-plugin,jenkinsci/git-plugin,jenkinsci/git-plugin,kzantow/git-plugin,recena/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,v1v/git-plugin,kzantow/git-plugin,jenkinsci/git-plugin,MarkEWaite/git-plugin,martinda/git-plugin,jenkinsci/git-plugin,martinda/git-plugin,kzantow/git-plugin,recena/git-plugin,jacob-keller/git-plugin,jacob-keller/git-plugin,MarkEWaite/git-plugin,v1v/git-plugin