[FIX] mrp: don't always display MO pickings button

Before this commit, Pickings button was always visible in manufacturing
order form view.
Now, it will be only visible if the MO has at least one picking.

Task #2047626

closes odoo/odoo#35455

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/mrp/views/mrp_production_views.xml

@@ -74,7 +74,7 @@
                             </div>
                         </button>
                         <button name="%(action_mrp_production_moves)d" type="action" string="Product Moves" class="oe_stat_button" icon="fa-exchange" attrs="{'invisible': [('state', 'not in', ('progress', 'done'))]}"/>
-                        <button type="object" name="action_view_mo_delivery" class="oe_stat_button" icon="fa-truck"  groups="base.group_user">
+                        <button type="object" name="action_view_mo_delivery" class="oe_stat_button" icon="fa-truck"  groups="base.group_user" attrs="{'invisible': [('delivery_count', '=', 0)]}">
                             <field name="delivery_count" widget="statinfo" string="Pickings"/>
                         </button>
                         <button class="oe_stat_button" name="action_see_move_scrap" type="object" icon="fa-arrows-v" attrs="{'invisible': [('scrap_count', '=', 0)]}">