Fix PHP 8.1 "strlen(null)" exception creating a Merchant in Phortune

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/phortune/provider/PhortunePaymentProvider.php:99]
```

Closes T15725

Test Plan: See steps in T15725

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15725

Differential Revision: https://we.phorge.it/D25528

src/applications/phortune/provider/PhortunePaymentProvider.php

@@ -96,7 +96,7 @@ abstract class PhortunePaymentProvider extends Phobject {
     array $issues);
 
   protected function renderConfigurationSecret($value) {
-    if (strlen($value)) {
+    if (phutil_nonempty_string($value)) {
       return str_repeat('*', strlen($value));
     }
     return '';