Fix PHP 8.1 strlen(null) in DiffusionView linkBrowse()

Summary:
Fix PHP 8.1 strlen(null) in DiffusionView linkBrowse().

Fixes T15542

Test Plan:

1. Go into a diff (eg https://my.phorge.site/D1234)
2. Click on the Revision Contents - History tab
3. Click on a base link (the short git revision ID)

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15542

Differential Revision: https://we.phorge.it/D25340