erp5_production_planning: set default value to 0

bt5/erp5_production_planning/SkinTemplateItem/portal_skins/erp5_production_planning/ProductionPlanning_view/matrixbox_quantity.xml

@@ -288,7 +288,7 @@
       <dictionary>
         <item>
             <key> <string>_text</string> </key>
-            <value> <string>python: cell.getQuantity() or 0 if cell else \'\'</string> </value>
+            <value> <string>python: cell.getQuantity() or 0 if cell else 0</string> </value>
         </item>
       </dictionary>
     </pickle>