give a default value to simplify TALES expressions

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35174 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/PropertySheet/SolverType.py

@@ -64,6 +64,7 @@ class SolverType:
          },
         { 'id':         'delivery_solver',
           'type':       'tokens',
+          'default':    (),
           'mode':       'w',
           'label':      'Delivery Solvers'
          },