Fix: Handle null content in PlanResource::formatContent method

- Fix deprecated warning when passing null to str_replace() function\n- Use null coalescing operator to provide empty string as default value\n- Comply with PHP 8.2+ type requirements

app/Http/Resources/PlanResource.php

@@ -80,7 +80,7 @@ class PlanResource extends JsonResource
      */
     protected function formatContent(): string
     {
-        $content = $this->resource['content'];
+        $content = $this->resource['content'] ?? '';
         
         $replacements = [
             '{{transfer}}' => $this->resource['transfer_enable'],