Fix an issue with "arc nrabch"
Fixes a minor issue from D14034. PHP doesn't like clone null;, and if you type a nonsense command like arc nbrhch (as I frequently do) we try to clone null here.
Instead, just return null; if no workflow matches. Clone otherwise.
- Ran arc nbrhcanc
- Ran arc branch
Reviewers: BYK, chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D14112