Fix PHP 8.1 "strlen(null)" exception for "/bin/conduit call" 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.

This patch also fixes two similar `strlen()` occurrences in the same source file.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/conduit/management/PhabricatorConduitCallManagementWorkflow.php:44]
```

Test Plan: Run `../phorge/bin/conduit call`: Get no `strlen()` error anymore but only expected output `Usage Exception: Specify a method to call with "--method".`

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/D25584

src/applications/conduit/management/PhabricatorConduitCallManagementWorkflow.php

@@ -41,19 +41,19 @@ final class PhabricatorConduitCallManagementWorkflow
     $viewer = $this->getViewer();
 
     $method = $args->getArg('method');
-    if (!strlen($method)) {
+    if (!phutil_nonempty_string($method)) {
       throw new PhutilArgumentUsageException(
         pht('Specify a method to call with "--method".'));
     }
 
     $input = $args->getArg('input');
-    if (!strlen($input)) {
+    if (!phutil_nonempty_string($input)) {
       throw new PhutilArgumentUsageException(
         pht('Specify a file to read parameters from with "--input".'));
     }
 
     $as = $args->getArg('as');
-    if (strlen($as)) {
+    if (phutil_nonempty_string($as)) {
       $actor = id(new PhabricatorPeopleQuery())
         ->setViewer($viewer)
         ->withUsernames(array($as))