R1:af71c990eee1

Test 0 and "" cases in Project Icon Config

Summary: Better validation for setting a default image in project.icon

Test Plan: Test adding `"0"` and `""` as image options in project.icon, see error.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18197