Fix PHP 8.1 "strlen(null)" exception viewing branches of non-existing Hg repo

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=HgViewBranches, ref.master=cb938d869c3f, ref.HgViewBranches=cb938d869c3f)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php:83]
```

Closes T15447

Test Plan: After applying this change, going to `/diffusion/1/branches/default/` shows an expected FilesystemException instead of the previous RuntimeException.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15447

Differential Revision: https://we.phorge.it/D25279