Fix PhabricatorRepository generateURI PHP 8.1 strlen(null) errors

Summary:
When viewing a repository in Diffusion, clicking on a folder (eg https://my.phorge.site/source/myrepo/browse/master/myfolder/) will generate multiple strlen(null) exceptions under PHP 8.1

Fix is to replace all the strlen() calls with phutil_nonempty_string()

Fixes T15532

Test Plan: View a folder in a repo in Diffusion. Eg https://my.phorge.site/source/myrepo/browse/master/myfolder/

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

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

Maniphest Tasks: T15532

Differential Revision: https://we.phorge.it/D25336