I've got to share some really useful, but yet undocumented feature that i've found out -- you know that if you select 'multiple' in a selector, you'll get 2 results, although if you untick multiple checkbox, it will catch only the first non-empty field.
If first field returns 'null' and second contains any data, it will return second field value.
Selectors must be the same type (e.g. text selector), otherwise it won't work.
EDIT1:I didn't check but just figured out, that you can put as much fields in a selector as you want, they must be separated by a comma, and it should still work as long as one of the fields contains data, it will be shown first.