GitSCM: remove the unnecessary fixNull for strings
GitSCM: remove the unnecessary fixNull for strings

There's no reason to implement a fixNull for strings, since Util already
provides one.

Signed-off-by: Jacob Keller <952d68dd7aa4b84e9f20381084cf63d55ea3c3b1@intel.com>

MarkEWaite/git-plugin,martinda/git-plugin,v1v/git-plugin,martinda/git-plugin,jenkinsci/git-plugin,v1v/git-plugin,jenkinsci/git-plugin,martinda/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,jenkinsci/git-plugin,v1v/git-plugin,jenkinsci/git-plugin