fix(theming): Return default theme if the user never selected a theme

[provokateurin]

Summary

While working on nextcloud/ocs_api_viewer#36 I found that if the user never selected a theme the data-themes attribute would only be an empty string instead of containing the default theme.
I'm not sure if the fallback also needs to be changed in other places, but this at least fixes the attribute problem.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

/backport to stable30

[provokateurin]

/backport to stable29

[provokateurin]

/backport to stable28

[susnux]

Makes sense, also tested and works fine.
I am not 100% this is intended, because "default" is not a theme but just an alias to select either dark or light depending on user agent, so empty attribute still means "default".