devel/arcanist-lib: Handle NULL argument passed to strlen()

Problem description:

• ArcanistDiffWorkflow.php:getDiffOntoTargets() tries to determine the current branch name by calling ArcanistGitAPI.php:getBranchName()

• getBranchName() returns NULL when working on a detached HEAD

• getDiffOntoTargets() calls strlen() with the branch name, which now fails with a NULL argument.

Approved by: grembo (maintainer)
Sponsored by: The FreeBSD Foundation