Fix PHP 8.1 "strlen(null)" exception for "/bin/herald rule" 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/herald/management/HeraldRuleManagementWorkflow.php:36]
```

Test Plan: Run `../phorge/bin/herald rule`: Get no `strlen()` error anymore but only expected output `Usage Exception: Specify a rule to edit with "--rule <id|monogram>".`

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/D25585

src/applications/herald/management/HeraldRuleManagementWorkflow.php

@@ -33,7 +33,7 @@ final class HeraldRuleManagementWorkflow
     $viewer = $this->getViewer();
 
     $rule_name = $args->getArg('rule');
-    if (!strlen($rule_name)) {
+    if (!phutil_nonempty_string($rule_name)) {
       throw new PhutilArgumentUsageException(
         pht('Specify a rule to edit with "--rule <id|monogram>".'));
     }