Fix PHP 8.1 "strlen(null)" exceptions trying to View Branches of a Diffusion repository

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=e4fd31ec024e), phorge(head=D25241, ref.master=b1edfea09bad, ref.D25241=b1edfea09bad)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/controller/DiffusionBranchTableController.php:29]

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=e4fd31ec024e), phorge(head=D25241, ref.master=b1edfea09bad, ref.D25241=b1edfea09bad)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php:33]

Closes T15414

Test Plan: Applied these two changes and at least got to (FilesystemException) Filesystem path "/var/repo/1/" does not exist. as an error message, instead of being stuck at (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15414

Differential Revision: https://we.phorge.it/D25241