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

src/applications/repository/management/PhabricatorRepositoryManagementMaintenanceWorkflow.php

@@ -42,7 +42,7 @@ final class PhabricatorRepositoryManagementMaintenanceWorkflow
     }
 
     $message = $args->getArg('start');
-    $is_start = (bool)strlen($message);
+    $is_start = $message !== null;
     $is_stop = $args->getArg('stop');
 
     if (!$is_start && !$is_stop) {