Fix EZP-23192: Cannot define array settings in default scope

> https://jira.ez.no/browse/EZP-23192

Some array settings like custom xsl files cannot be defined in `default` scope with semantic configuration. This forces bundle developers who relies these kind of settings to provide additional installation steps (e.g. for [TagsBundle](https://github.com/netgen/TagsBundle/blob/master/Resources/doc/INSTALL.md#edit-configuration)).
Workaround is to implement a compiler pass that does the job (e.g. [EmbedTagBundle](https://github.com/lolautruche/EmbedTagBundle/blob/master/DependencyInjection/Compiler/XslRegisterPass.php), but it still doesn't use semantic config validation and proper processing/mapping.

This is due to default scope not being considered as a siteaccess when merging values across scopes).

This PR fixes it.