title: Disable the empty option in Django ModelForm
body: <p>I use <code>widget=forms.CheckboxSelectMultiple</code> in a <code>ModelForm</code> subclass to change the field display to checkboxes. The choices do become checkboxes but Django adds a first "empty" option with <code>---------</code> as the content.</p>
  
  <p>This behavior is OK when displaying the field using the <code>Select</code> widget, because it force the user to explicitly choose an option. However when using checkboxes it becomes useless and rather confusing. How can I disable it?</p>