Fix PHP 8.1 "strlen(null)" exception on History page of Diffusion repo after renaming default branch

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=arcpatch-D25250, ref.master=18554ea76ceb, ref.arcpatch-D25250=fade4603a799), phorge(head=master, ref.master=e11c5486c92b)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/repository/xaction/PhabricatorRepositoryDefaultBranchTransaction.php:25]

Closes T15434

Test Plan: After applying this change, after changing the name of the (previously empty) default branch of a Diffusion repository, and then going to the repository History at /diffusion/<someID>/manage/history/, the page renders correctly in the web browser instead of showing an exception.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15434

Differential Revision: https://we.phorge.it/D25265