Fix a PHP 8.1 repository marker issue in Mercurial

Summary:
Ref T13588. "arc-ls-markers" emits a "branch-state" marker so callers can identify which branch is active in the working copy.

This marker doesn't have an associated commit, so trying to generate a display name fails under stricter PHP 8.1 rules when we try to substr(null, ...).

Don't attempt to generate a display name for markers with no commit hash.

Test Plan:

• Ran arc branches under PHP 8.1 in a Mercurial repository.
• Before: fatal.
• After: sensible output.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21825