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.
Merge pull request #940 from ezsystems/fix_EZP-23192_arraySettingsDefaultScope

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