title: How to insert an empty row in a combobox filled by linq?
body: <p>I have a list of Items, given by Linq from the DB.
  Now I filled a ComboBox with this list.</p>
  
  <p>How I can get an empty row in it?</p>
  
  <p>My problem is that in the list the values aren't allowed to be null, so I can't easily add a new empty item.</p>