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

[jrm]

Summary

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.

[jrm]

A more general fix was made.