[FIX] mrp: force default values for dropped fields

Those values were dropped at rev 9b343581 but
need to be forced to empty so older databases
get fixed when upgrading

addons/mrp/mrp_workflow.xml

@@ -57,6 +57,9 @@
             <field name="act_from" ref="prod_act_confirmed"/>
             <field name="act_to" ref="prod_act_ready"/>
             <field name="signal">moves_ready</field>
+            <field name="trigger_model" eval="False"/> <!-- Force empty -->
+            <field name="trigger_expr_id" eval="False"/> <!-- Force empty -->
+            <field name="condition">True</field> <!-- Force default -->
         </record>
         <record id="prod_trans_ready_in_production" model="workflow.transition">
             <field name="act_from" ref="prod_act_ready"/>