return a boolean otherwise this proxy field cannot be used in a listbox

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@44513 20353a03-c40f-0410-a6d1-a30d3c3de9de

bt5/erp5_base/SkinTemplateItem/portal_skins/erp5_base/BankAccount_viewFieldLibrary/my_stop_date.xml

@@ -127,7 +127,7 @@
       <dictionary>
         <item>
             <key> <string>_text</string> </key>
-            <value> <string>python: context.hasStopDate() and context.getStopDate()</string> </value>
+            <value> <string>python: context.hasStopDate() and context.getStopDate() or None</string> </value>
         </item>
       </dictionary>
     </pickle>

bt5/erp5_base/bt/revision

-963
\ No newline at end of file
+964
\ No newline at end of file