bug #34877 [TwigBundle] fix findTemplate() to return null

Twig3 FilesystemLoader::findTemplate() should return `string|null`
instead of Twig2 `string|null|false`: see
<https://github.com/twigphp/Twig/blob/3.x/src/Loader/FilesystemLoader.php#L167>

Returning `null` fixes `exists()` of Twig 3 FilesystemLoader without
breaking Twig 2 (which expected `null` or `false` for not found
templates).

Change the test to assert `null` instead of `false`.

src/Symfony/Bundle/TwigBundle/Loader/FilesystemLoader.php

@@ -96,7 +96,7 @@ protected function findTemplate($template, $throw = true)
                 throw $twigLoaderException;
             }
 
-            return false;
+            return null;
         }
 
         return $this->cache[$logicalName] = $file;

src/Symfony/Bundle/TwigBundle/Tests/Loader/FilesystemLoaderTest.php

@@ -123,6 +123,6 @@ public function testTwigSoftErrorIfTemplateDoesNotExist()
 
         $method = new \ReflectionMethod('Symfony\Bundle\TwigBundle\Loader\FilesystemLoader', 'findTemplate');
         $method->setAccessible(true);
-        $this->assertFalse($method->invoke($loader, 'name.format.engine', false));
+        $this->assertNull($method->invoke($loader, 'name.format.engine', false));
     }
 }