[JENKINS-67491] Do not fail on users disabled by auth provider
[JENKINS-67491] Do not fail on users disabled by auth provider

Matches the stack trace in the bug report.

Does not protect the other calls to User.get().

jenkinsci/git-plugin,martinda/git-plugin,jenkinsci/git-plugin,martinda/git-plugin,jenkinsci/git-plugin,jenkinsci/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,martinda/git-plugin