Fix for #17103 dojox.form.CheckedMultiSelect menu doesn't have checkboxes

This fix in dojox/form/CheckedMultiSelect.js is for the part of #17103 which has to do with the menu not displaying the checkboxes.  I have also updated the mvc test which tests the problem which introduced the other problem (the empty combobox) reported in #17103, to be sure the fix for this does not regress that fix.