erp5_simulation: fix Rule_view crashes if a Category Membership Divergence...

erp5_simulation: fix Rule_view crashes if a Category Membership Divergence Tester tested property is not set

bt5/erp5_simulation/SkinTemplateItem/portal_skins/erp5_simulation/Rule_view/listbox_tested_property_list.xml

@@ -118,7 +118,7 @@
       <dictionary>
         <item>
             <key> <string>_text</string> </key>
-            <value> <string>python: [x for x in cell.getTestedPropertyList() if x is not None]</string> </value>
+            <value> <string>python: cell.getTestedPropertyList() or \'\'</string> </value>
         </item>
       </dictionary>
     </pickle>