Fix PHP 8.1 "strlen(null)" exceptions trying to browse Diffusion repository code

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=diffusionBrowseCode, ref.master=108cbcd09bd3, ref.diffusionBrowseCode=108cbcd09bd3)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/controller/DiffusionBrowseController.php:25]

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=diffusionBrowseCode, ref.master=108cbcd09bd3, ref.diffusionBrowseCode=108cbcd09bd3)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/diffusion/conduit/DiffusionBrowseQueryConduitAPIMethod.php:40]

Closes T15462

Test Plan: After applying these two changes, get next expected exceptions about RuntimeException: file_exists(): Passing null to parameter #1 ($filename) of type string is deprecated and CommandException: Command failed with error #128! fatal: detected dubious ownership in repository at '/var/repo/1', but no RuntimeException about strlen() anymore.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15462

Differential Revision: https://we.phorge.it/D25286