Make standard fields more liberal about interpreting empty strings

Summary:
Fixes T3867. We currently show more empty custom field values on task detail pages than we should, for at least two reasons:

• <select /> fields with an empty string option store "", but users reasonably expect this to mean "no value".
• Old fields may have stored empty strings, and migrated forward.

This fix generally aligns behavior with user expectations. We could get more extreme about not storing "" in the database, but I think this is generally a less surpsing fix.

Test Plan: Made a select with a "" : "None" option, selected it, saw it vanish from task detail screen.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3867

Differential Revision: https://secure.phabricator.com/D7105