Fix PHP 8.1 "strlen(null)" exception which blocks rendering a Diffusion repository page

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15421

Test Plan:
Applied this change, create a repository, and afterwards the repository page at `/diffusion/9/` is shown with its name and "Empty Repository - This repository does not have any commits yet."

- have a Diffusion repository with Staging Area and Automation already setup
- create a Differential patch from arc
- press the Land button on the UI and write random things inside

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15421

Differential Revision: https://we.phorge.it/D25247