Validate select/option settings more strictly when reading them

Summary:
Ref T4103. If the database has "" (empty string) for select/option settings, we can let that value be effective in the UI right now.

One consequence is that timestamps can vanish from the UI.

Instead, be stricter and discard it as an invalid value.

Test Plan:

• Forced time-format setting to ''.
• Saw timestamps vanish before change.
• Saw timestamps return to the default value after change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16047