[8.x] Fix stringable docblock (#37835)

Add missing `null` default value on `Blade@stringable` façade docblock

src/Illuminate/Support/Facades/Blade.php

@@ -26,7 +26,7 @@
  * @method static void withDoubleEncoding()
  * @method static void withoutComponentTags()
  * @method static void withoutDoubleEncoding()
- * @method static void stringable(string|callable $class, callable|null $handler)
+ * @method static void stringable(string|callable $class, callable|null $handler = null)
  *
  * @see \Illuminate\View\Compilers\BladeCompiler
  */