[FIX] mrp: bypass _plan_workorder in case of no workorder

The _plan_workorder function is callable for the production order list
view. Calling it on production without workorder make no sense and can
even crash.
This commit bypass it in those case

Task : 2278147

addons/mrp/models/mrp_production.py

@@ -1113,6 +1113,8 @@ class MrpProduction(models.Model):
         """
         self.ensure_one()
 
+        if not self.workorder_ids:
+            return
         # Schedule all work orders (new ones and those already created)
         qty_to_produce = max(self.product_qty - self.qty_produced, 0)
         qty_to_produce = self.product_uom_id._compute_quantity(qty_to_produce, self.product_id.uom_id)