erp5_production_planning: quantity should not be negative

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

@@ -249,7 +249,7 @@
                 </item>
                 <item>
                     <key> <string>start</string> </key>
-                    <value> <string></string> </value>
+                    <value> <int>0</int> </value>
                 </item>
                 <item>
                     <key> <string>title</string> </key>