Added default return type array for getCategories in CampaignCategory

* Update CampaignCategory.php

Removed array typecast on getCategories. If a category has no sub categories we where getting errors like this:
Symfony\Component\Debug\Exception\FatalThrowableError: Return value of Hypeit\TradeTracker\Model\CampaignCategory::getCategories() must be of the type array, null returned

* Update CampaignCategory.php

Make response getCategories backwards compatible

* Update CampaignCategory.php

return type restriction on getCategories

Author: lvdhoorn

src/Model/CampaignCategory.php

@@ -64,7 +64,7 @@ public function addCategory(CampaignCategory $category)
      */
     public function getCategories(): array
     {
-        return $this->categories;
+        return $this->categories ?? [];
     }
 
     /**