Fix: When the template ID changes, don't try to set default fields where the old template didn't have a default value in the first place.
 
 Previously, this incorrect behaviour was possible: when both the old and the new template had an unset, default field without an actual default value, and the field was displaying a placeholder (e.g. an empty string or some such), upon changing the template that placeholder would become the new, custom value of that field. In effect, a custom value would show up out of nowhere.
  
  Instead, when a field has neither a custom nor a default setting, just leave it as null and let the field handler (knownMenuFields) deal with it.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@941534 b8457f37-d9ea-0310-8a92-e5e31aec5664