Implement better messaging for Assert for css in libraries discovery

Problem/Motivation

Today I found out that I had the following structure in a *.libraries.yml.

global-styling:
  css:
    component:
      assets/css/components/mycomponent1/mycomponent1.css:
      assets/css/components/mycomponent2/mycomponent2.css: {}

In Drupal 11, that missing {} translates to a bricked site. I could not even login using drush uli.

The error message that was displayed was
AssertionError: CSS must be nested under a category. See https://www.drupal.org/node/2274843. in assert() (line 128 of /var/www/html/docroot/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php)..

The returns in LibraryDiscoveryParser::validateCssLibrary should be updated so that is_array is not the only test.

Steps to reproduce

1. Open a library and remove the {}.
2. Site is now bricked

Proposed resolution

The current messages implies that a libray is missing one of the 5 SMACSS keywords. The message does not state what the actual issue is; the yml is malformed.

Remaining tasks

TBD

User interface changes

TBD

Introduced terminology

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD