Fix PHP 8.1 "strlen(null)" exception for "/bin/files migrate" without parameter

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/files/management/PhabricatorFilesManagementMigrateWorkflow.php:65]
```

Test Plan: Run `../phorge/bin/files migrate`: Get no `strlen()` error anymore but only expected output `Usage Exception: Specify an engine to migrate to with `--engine`. Use `files engines` to get a list of engines.`

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25594

src/applications/files/management/PhabricatorFilesManagementMigrateWorkflow.php

@@ -62,7 +62,7 @@ final class PhabricatorFilesManagementMigrateWorkflow
     // it as though it is configured to use the specified location.
 
     $local_disk_source = $args->getArg('local-disk-source');
-    if (strlen($local_disk_source)) {
+    if (phutil_nonempty_string($local_disk_source)) {
       $path = Filesystem::resolvePath($local_disk_source);
       try {
         Filesystem::assertIsDirectory($path);