[FIX] mrp: do not erase move raw at onchange without bom

With this commit, the move raw refresher onchange is bypassed in case of
empty BoM. As the manufacturing user can adds some components on the fly
to create a non manufactured product, changing the quantity to produce
or the unit of measur will erase all the move raw and recreate them from
the BoM lines. As no boms are set, the components are just deleted.

Task : 2278147

addons/mrp/models/mrp_production.py

@@ -604,6 +604,8 @@ class MrpProduction(models.Model):
 
     @api.onchange('bom_id', 'product_id', 'product_qty', 'product_uom_id')
     def _onchange_move_raw(self):
+        if not self.bom_id and not self._origin.product_id:
+            return
         # Clear move raws if we are changing the product. In case of creation (self._origin is empty),
         # we need to avoid keeping incorrect lines, so clearing is necessary too.
         if self.product_id != self._origin.product_id: