#3626982: Keep the runtime theme registry usable after destruct

Luca Rossirequested to merge
issue/drupal-3626982:3626982-themeregistry-is-empty into main

Empties the runtime registries in Registry::destruct() after destructing them, so a later getRuntime() in the same process builds a fresh ThemeRegistry that loads from the cache. Adds a kernel test that fails without the fix.

Closes #3626982

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