Library CSS definition should not fail is no CSS files defined

What I'm thinking is something like this:

diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 78097760f4..44aa8f1c47 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -654,16 +654,16 @@ protected function resolveThemeAssetPath($theme_path, $overriding_asset) {
    *     - 2 if the library definition specifies files as an array
    */
   public static function validateCssLibrary($library) {
-    $categories = [];
     // Verify options first and return early if invalid.
     foreach ($library as $category => $files) {
       if (!is_array($files)) {
         return 2;
       }
-      $categories[] = $category;
-      foreach ($files as $options) {
-        if (!is_array($options)) {
-          return 1;
+      if (!is_null($files)) {
+        foreach ($files as $options) {
+          if (!is_array($options)) {
+            return 1;
+          }
         }
       }
     }

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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Fixed pipeline issues (PhpCodingStandard + Javascript linting) and have updated the MR, please review it.

Thanks!

Changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to our policies.

test-only feature passes when it should be failing so test coverage needs to be expanded.