Handle unset keys in \Drupal\role_theme_switcher\Theme\RoleThemeSwitcherNegotiator::applies

Problem/Motivation

The config entity is not always complete, i.e. if a new role gets created and a config script e.g. sets something like drush -y config:set role_theme_switcher.settings roles.newrole.theme gin, then the admin_theme key is missing in that case. That creates warnings on screen and in the log.

Proposed resolution

Use a default, if a value is not defined.