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 instead of global defaults until you save anything.
  • After change: properly shows global defaults from the start.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16112