Fix PHP 8.1 "strlen(null)" exception for "/bin/nuance import" 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/nuance/management/NuanceManagementWorkflow.php:8]
```

Test Plan: Run `../phorge/bin/nuance import`: Get no `strlen()` error anymore but only expected output `Usage Exception: Specify a source with --source.`

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/D25580

src/applications/nuance/management/NuanceManagementWorkflow.php

@@ -5,7 +5,7 @@ abstract class NuanceManagementWorkflow
 
   protected function loadSource(PhutilArgumentParser $argv, $key) {
     $source = $argv->getArg($key);
-    if (!strlen($source)) {
+    if (!phutil_nonempty_string($source)) {
       throw new PhutilArgumentUsageException(
         pht(
           'Specify a source with %s.',