Guard against null permission_ids in getModulesByPermissionSlug()

modules.permission_ids is NULL for module 8 ("Szerviz kategoriak") on every
environment, and PHP 8.1 deprecates passing null to explode(). The notice was
only visible on prod because its index.php still has the stock
error_reporting(-1), whereas dev was previously changed to
E_ALL & ~E_DEPRECATED & ~E_WARNING & ~E_NOTICE.

- Skip rows whose permission_ids is null or empty. Behaviour-preserving:
  explode(',', null) returned array(''), which matched no permission id, so
  such modules were already excluded. Verified by simulating old vs new
  against the real module rows across six permission-id values including 0,
  '1' and 'admin' - identical results.
- Return early when the slug matches no permission, rather than dereferencing
  null on the next line.

Does not address the wider issue that prod runs ENVIRONMENT='development'
with display_errors=1, so any notice is rendered to real visitors.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

application/models/Module_model.php

@@ -164,10 +164,25 @@ public function getModulesByPermissionSlug($permissionSlug){
     $this->load->model('Module_model');
     $selectedPermission = $this->Module_model->getPermissionBySlug($permissionSlug);
 
+    // An unknown slug yields no permission at all; without this the line below
+    // dereferences null. No permission means no modules.
+    if(!$selectedPermission){
+        return array();
+    }
+
     $query = $this->db->query('SELECT * FROM modules;');
     $results = $query->result();
     $resultArray = array();
     foreach($results as $resultItem){
+        // modules.permission_ids is NULL for at least one row (module 8,
+        // 'Szerviz kategoriak') on every environment. Passing null to explode()
+        // is deprecated in PHP 8.1 and emits a notice. A module with no
+        // permissions is visible to nobody, so skipping it keeps the previous
+        // behaviour - explode(',', null) returned array('') which matched nothing.
+        if($resultItem->permission_ids === null || $resultItem->permission_ids === ''){
+            continue;
+        }
+
         $permissionIdsArray = explode(',', $resultItem->permission_ids);
         if(in_array($selectedPermission->permission_id, $permissionIdsArray)){
             $resultArray[] = $resultItem;