Fix an issue with "arc nrabch"

Summary:
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.

Test Plan:

• Ran arc nbrhcanc
• Ran arc branch

Reviewers: BYK, chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14112