sakonn's patch from #23 as a starting point

Closes #3359497

core/lib/Drupal/Core/Asset/CssCollectionOptimizerLazy.php

@@ -145,7 +145,9 @@ public function optimizeGroup(array $group): string {
       // Ensure license information is available as a comment after
       // optimization.
       if ($css_asset['license'] !== $current_license) {
-        $data .= "/* @license " . $css_asset['license']['name'] . " " . $css_asset['license']['url'] . " */\n";
+        // It is possible to have a license name with no URL.
+        $url = $css_asset['license']['url'] ?? 'no URL';
+        $data .= "/* @license " . $css_asset['license']['name'] . " " . $url . " */\n";
       }
       $current_license = $css_asset['license'];
       $data .= $this->optimizer->optimize($css_asset);