Fix PHP 8.1 "strlen(null)" exception for "/bin/files encode" without --as 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/PhabricatorFilesManagementEncodeWorkflow.php:45]
```

Test Plan: Run `../phorge/bin/files encode F1`: Get no `strlen()` error anymore but only expected output `Usage Exception: Use --as <format> to select a target encoding format. Available formats are: aes-256-cbc, rot13, raw.`

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/D25595

src/applications/files/management/PhabricatorFilesManagementEncodeWorkflow.php

@@ -42,7 +42,7 @@ final class PhabricatorFilesManagementEncodeWorkflow
     $format_list = implode(', ', $format_list);
 
     $format_key = $args->getArg('as');
-    if (!strlen($format_key)) {
+    if (!phutil_nonempty_string($format_key)) {
       throw new PhutilArgumentUsageException(
         pht(
           'Use --as <format> to select a target encoding format. Available '.