Fix crash in polygon "create vertices" called from CMB.
- This fixes
smtk::attribute::Attribute::isValid()
to test each of its items as well as its references. - This fixes the templated
isInvalid
function called during validity testing to better handle the case when the maximum number of values in the item's definition is zero. - Don't require attribute validity in the
qtModelOperationWidget
initialization code as items without defaults may not be valid at that point.