R1:d68b2cc0e4c4

Fix construction of default settings for users with no settings at all

Summary:
Ref T11098. Users with at least one setting set correctly fall back to the defaults, but users with no settings at all currently do not.

Make them fall back to global defaults properly.

Test Plan:
- Set global defaults to some non-default setting.
- Completely delete a user's settings.
- `bin/cache purge --purge-all` or `--purge-user`.
- View settings as the user.
- Before change: showed hard-coded defaults…