Check for model key before expecting it

Solves issues where the field may not have a `model` key, such as in `checklist_dependency` fields. This fixes the issue found in PermissionManager here - Laravel-Backpack/PermissionManager#291

Author: olipayne

src/app/Library/CrudPanel/Traits/FakeFields.php

@@ -39,7 +39,7 @@ public function compactFakeFields($requestInput, $model = false, $fields = [])
                 $isFakeField = $field['fake'] ?? false;
 
                 // field is represented by the subfields
-                if (isset($field['subfields']) && $field['model'] === get_class($model)) {
+                if (isset($field['subfields']) && isset($field['model']) && $field['model'] === get_class($model)) {
                     foreach ($field['subfields'] as $subfield) {
                         $subfieldName = Str::afterLast($subfield['name'], '.');
                         $isSubfieldFake = $subfield['fake'] ?? false;