[Routing] AnnotationDirectoryLoader::load() may return null

Closes #45259

Author: mhujer

Loader/AnnotationDirectoryLoader.php

@@ -25,7 +25,7 @@ class AnnotationDirectoryLoader extends AnnotationFileLoader
     /**
      * @throws \InvalidArgumentException When the directory does not exist or its routes cannot be parsed
      */
-    public function load(mixed $path, string $type = null): RouteCollection
+    public function load(mixed $path, string $type = null): ?RouteCollection
     {
         if (!is_dir($dir = $this->locator->locate($path))) {
             return parent::supports($path, $type) ? parent::load($path, $type) : new RouteCollection();

Tests/Loader/AnnotationDirectoryLoaderTest.php

@@ -99,6 +99,14 @@ public function testLoadFileIfLocatedResourceIsFile()
         $this->loader->load(__DIR__.'/../Fixtures/AnnotatedClasses/FooClass.php');
     }
 
+    public function testLoadAbstractClass()
+    {
+        $this->reader->expects($this->never())->method('getClassAnnotation');
+        $this->reader->expects($this->never())->method('getMethodAnnotations');
+
+        $this->loader->load(__DIR__.'/../Fixtures/AnnotatedClasses/AbstractClass.php');
+    }
+
     private function expectAnnotationsToBeReadFrom(array $classes)
     {
         $this->reader->expects($this->exactly(\count($classes)))