[9.x] Fix whenPivotLoaded(As) api resource methods when using Eloquent strict mode (#44792)

* Add failing test

* Use isset method

* Fix linting

Author: gdebrauwer

src/Illuminate/Http/Resources/ConditionallyLoadsAttributes.php

@@ -291,7 +291,7 @@ protected function whenPivotLoadedAs($accessor, $table, $value, $default = null)
         }
 
         return $this->when(
-            $this->resource->$accessor &&
+            isset($this->resource->$accessor) &&
             ($this->resource->$accessor instanceof $table ||
             $this->resource->$accessor->getTable() === $table),
             ...[$value, $default]

tests/Integration/Http/ResourceTest.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Tests\Integration\Http;
 
+use Illuminate\Database\Eloquent\Model;
 use Illuminate\Foundation\Http\Middleware\ValidatePostSize;
 use Illuminate\Http\Exceptions\PostTooLargeException;
 use Illuminate\Http\Request;
@@ -443,6 +444,33 @@ public function testResourcesMayHaveOptionalPivotRelationships()
         ]);
     }
 
+    public function testResourceDoesNotThrowErrorWhenUsingEloquentStrictModeAndCheckingOptionalPivotRelationship()
+    {
+        Model::shouldBeStrict(true);
+
+        Route::get('/', function () {
+            $post = new Post(['id' => 5]);
+            (function () {
+                $this->exists = true;
+                $this->wasRecentlyCreated = false;
+            })->bindTo($post)();
+
+            return new PostResourceWithOptionalPivotRelationship($post);
+        });
+
+        $response = $this->withoutExceptionHandling()->get(
+            '/', ['Accept' => 'application/json']
+        );
+
+        $response->assertStatus(200);
+
+        $response->assertExactJson([
+            'data' => [
+                'id' => 5,
+            ],
+        ]);
+    }
+
     public function testResourcesMayHaveOptionalPivotRelationshipsWithCustomAccessor()
     {
         Route::get('/', function () {