Fix PHP 8.1 "strlen(null)" exception trying to stop maintenance mode of Diffusion 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.

Use isset instead to check if the argument "start" was passed to the CLI command.

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=repoMaintStartHistory, ref.master=1c098c273d06, ref.repoMaintStartHistory=0a4a34143528)
  #0 PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/repository/management/PhabricatorRepositoryManagementMaintenanceWorkflow.php:45]

Closes T15476

Test Plan: After applying this change, executing ./bin/repository maintenance --stop R1 on the CLI shows Took repository "R1" out of maintenance mode. as expected, instead of an exception.

Reviewers: O1 Blessed Committers, valerio.bozzolan, aklapper, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

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

Maniphest Tasks: T15476

Differential Revision: https://we.phorge.it/D25296