(Multi|)RelationField: make portal_type optional.

Code tolerates this with no change.
Allows jumping to an existing relation without having to hard-code
(potentially a lot of) portal types.

product/ERP5Form/MultiRelationField.py

@@ -109,7 +109,7 @@ class MultiRelationStringFieldWidget(Widget.LinesTextAreaWidget,
                              description=(
       "The method to call to set the relation. Required."),
                              default="",
-                             required=1)
+                             required=0)
 
   allow_creation = fields.CheckBoxField('allow_creation',
                              title='Allow Creation',