erp5_base: Make sure we do not acquire start_date in listbox_stop_date

for Person_viewAssignment form

bt5/erp5_base/SkinTemplateItem/portal_skins/erp5_base/Person_viewAssignment/listbox_stop_date.xml

@@ -55,7 +55,9 @@
               <dictionary>
                 <item>
                     <key> <string>default</string> </key>
-                    <value> <string></string> </value>
+                    <value>
+                      <persistent> <string encoding="base64">AAAAAAAAAAI=</string> </persistent>
+                    </value>
                 </item>
                 <item>
                     <key> <string>editable</string> </key>
@@ -108,4 +110,17 @@
       </dictionary>
     </pickle>
   </record>
+  <record id="2" aka="AAAAAAAAAAI=">
+    <pickle>
+      <global name="TALESMethod" module="Products.Formulator.TALESField"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_text</string> </key>
+            <value> <string>python: cell.getStopDate() if cell.hasStopDate() else None</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
 </ZopeData>