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

src/applications/project/icon/PhabricatorProjectIconSet.php

@@ -262,7 +262,8 @@ final class PhabricatorProjectIconSet
 
       $is_disabled = idx($value, 'disabled');
 
-      if (idx($value, 'image')) {
+      $image = idx($value, 'image');
+      if ($image !== null) {
         $builtin = idx($value, 'image');
         $builtin_map = id(new PhabricatorFilesOnDiskBuiltinFile())
           ->getProjectBuiltinFiles();