LibraryHooks::libraryInfoBuild() assumes all libraries have CSS

Overview

\Drupal\experience_builder\Hook\LibraryHooks::libraryInfoBuild assumes all libraries have `css` key. The extensions library does not.

# Warning: Undefined array key "css" in Drupal\experience_builder\Hook\LibraryHooks->libraryInfoBuild() (line 124 of modules/contrib/experience_builder/src/Hook/LibraryHooks.php).

    foreach ($libraries as $library_name => &$library) {
      foreach ($libraries[$library_name]['css'] as $files) {
        foreach ($files as $filename => $file_definition) {

Proposed resolution

I think \Drupal\experience_builder\Hook\LibraryHooks::buildExtensionLibraries needs to add css

    if (!empty($xb_extensions)) {
      $libraries['extensions'] = [
        'dependencies' => $xb_extensions,
        'css' => []
      ];
    }

Or the other code cannot assume css

User interface changes