Make Twig debug comments optional for empty markup.

@Cilefen - what happens with the 10.1.x? Is this going to be ported back?

11.x-dev is the feature branch.

https://www.drupal.org/about/core/blog/new-drupal-core-branching-scheme-... →

This looks nice.
Here is a reroll against 11.x. I only added a small test case if that's ok.

PS: As per ✨ Allow theme preprocess hooks to abort rendering Active , making it non disruptive by using a switch is a good idea, but could also make it harder to solve the other issue I was trying to solve there. Outputting debug markup when the theme output is completely empty prevents a proper emptiness check.
Any opinions on this? I'm kinda stuck there. Perhaps the issue is not scoped very well there.

Also helpful fix from https://drupal.slack.com/archives/C1BMUQ9U6/p1705070170229879

diff --git a/web/core/lib/Drupal/Core/Render/Renderer.php b/web/core/lib/Drupal/Core/Render/Renderer.php
--- a/web/core/lib/Drupal/Core/Render/Renderer.php
+++ b/web/core/lib/Drupal/Core/Render/Renderer.php
@@ -836,7 +851,7 @@ protected function addDebugOutput(array $elements, bool $is_cache_hit, array $pr
         }
         $prefix .= "\n-->";
       }
-      if (!empty($debug_item['#cache']['max-age'])) {
+      if (isset($debug_item['#cache']['max-age'])) {
         $prefix .= "\n<!-- " . $name_prefix . " MAX-AGE: " . $debug_item['#cache']['max-age'] . " -->";
       }
     }

Opened MR for 11.x
Attaching static patch for composer as well.

* Proper static composer patch sry

Kind of change will probably need a change record.