Fix PHP 8.1 "strlen(null)" exception on History page of Diffusion repo after setting maintenance mode

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=master, ref.master=1c098c273d06)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/repository/xaction/PhabricatorRepositoryMaintenanceTransaction.php:28]

Closes T15475

Test Plan:
Play a bit with maintenance messages:

./bin/repository maintenance --start "Start"      R1
./bin/repository maintenance --start "Start Yeah" R1
./bin/repository maintenance --stop               R1

Then visit /diffusion/1/manage/history/ and look at Diffusion put this repository into maintenance mode. as expected, instead of an exception.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15475

Differential Revision: https://we.phorge.it/D25295