[FIX] mrp: create a bom with positive quantity

Creation of production order from a bom with 0 quantity to produce will
fail. As there is already a constraint on the quantity to produce on MO,
this commit adds the same constraint on BoM.

closes odoo/odoo#48485

X-original-commit: 1330500885ae096372ab32fba4ffc9db252dd2d4
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/mrp/models/mrp_bom.py

@@ -74,6 +74,10 @@ class MrpBom(models.Model):
         required=True
     )
 
+    _sql_constraints = [
+        ('qty_positive', 'check (product_qty > 0)', 'The quantity to produce must be positive!'),
+    ]
+
     @api.onchange('product_id')
     def onchange_product_id(self):
         if self.product_id: