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