Fix PHP 8.1 "strlen(null)" exception rendering a commit in Diffusion when bugtraq.url = null

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.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=97e163187418), phorge(head=diffusionBrowseCommit, ref.master=9b99123ff931, ref.diffusionBrowseCommit=9b99123ff931)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/controller/DiffusionCommitController.php:925]
```

Closes T15464

Test Plan: After applying this change, selecting the last commit on http://phorge.localhost/diffusion/query/all/ correctly renders the commit page (e.g. `/R1:1234567890abcdef1234567890abcdef12345678`)

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15464

Differential Revision: https://we.phorge.it/D25288