fix(phpstan): code improvements and resolve warnings

Reviewed code and made several improvements which provide better error handling, code quality, and resolved warnings. The changes include:

• Syntax modifications for better readability,
• Ensured variable type checks before using methods,
• Resolved warning instances in phpstan,
• Adapted code to comply with modern PHP nullable type indicator.

Alongside, made version update in phive/phars.xml. Also, unnecessary comments or instances are removed for a cleaner code.

Related: #16 (closed)

src/QUI/ERP/Coupons/Events.php

@@ -44,6 +44,10 @@ class Events
      */
     public static function onPackageSetup(QUI\Package\Package $Package): void
     {
+        if ($Package->getName() !== 'quiqqer/coupons') {
+            return;
+        }
+
         try {
             self::createProductFields();
         } catch (Exception $Exception) {