Treat 0 as a valid data element in custom_field and if_custom_field tags. Throw warning if field doesn't exist to rid us of error suppression. Thanks gaekwad

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5585 2fea6a4d-a838-0410-917b-93a53c48e9d2