Listbox: Use an hidden field so that validator knows that listbox was empty

This way validator scripts will be called even for empty listboxs, and this
should also remove ERP5Form/Form.py:validate_all KeyError : 'Field listbox is
not present in request object.' messages from the logs

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/ListBox_asHTML.xml

@@ -503,6 +503,8 @@
                 <span i18n:translate="" i18n:domain="ui">\n
                   No result.\n
                 </span>\n
+                <input type="hidden" value="1" name="listbox_empty:boolean"\n
+                       tal:attributes="name string:${field_prefix}${field_id}_empty:boolean" />\n
               </td>\n
             </tr>\n
           </tbody>\n

product/ERP5/bootstrap/erp5_xhtml_style/bt/revision

-1089
\ No newline at end of file
+1090
\ No newline at end of file

product/ERP5Form/ListBox.py

@@ -2762,11 +2762,15 @@ class ListBoxValidator(Validator.Validator):
 
         result = {}
         error_result = {}
-        try:
-          listbox_uids = REQUEST['%s_uid' % field.id]
-        except KeyError:
-          raise KeyError('Field %s is not present in request object.'
-                         % field.id)
+        listbox_empty = REQUEST.get('%s_empty' % field.id, False)
+        if listbox_empty:
+          listbox_uids = []
+        else:
+          try:
+            listbox_uids = REQUEST['%s_uid' % field.id]
+          except KeyError:
+            raise KeyError('Field %s is not present in request object.'
+                           % field.id)
         select = field.get_value('select')
         if select:
           selected_uid_set = set(REQUEST.get('uids', ()))