Fix PHP 8.1 "strlen(null)" exception on SVN repo page with no branches

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=b325304b6e52), phorge(head=SvnRepoMain, ref.master=cb938d869c3f, ref.SvnRepoMain=cb938d869c3f)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/controller/DiffusionController.php:156]
```

Closes T15446

Test Plan: After applying this change, going to a newly created SVN repository at `/diffusion/1/`, the page renders, showing the repository as Inactive and also "Empty Repository - This repository does not have any commits yet." as expected.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15446

Differential Revision: https://we.phorge.it/D25278