[12.x] add docs for @hasstack (#10913)

* add docs for `@hasstack`

document laravel/framework#57788

* Update blade.md

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: browner12

blade.md

@@ -1855,6 +1855,16 @@ If you would like to prepend content onto the beginning of a stack, you should u
 @endprepend
 ```
 
+The `@hasstack` directive may be used to determine if a stack is empty:
+
+```blade
+@hasstack('list')
+    <ul>
+        @stack('list')
+    </ul>
+@endif
+```
+
 <a name="service-injection"></a>
 ## Service Injection