Do not delete purchase request with a purchase line when supply stock

The design does not ensure that draft purchase request can not be linked to a
purchase line.

Closes #12974
(grafted from 88d4acee578219e8c193b56ceb5a4f3bf3d7b1aa)

--HG--
branch : 6.0

modules/stock_supply/purchase_request.py

@@ -135,6 +135,7 @@ class PurchaseRequest(metaclass=PoolMeta):
         products = set(products)
         reqs = cls.search([
                 ('state', '=', 'draft'),
+                ('purchase_line', '=', None),
                 ('company', '=', company.id),
                 ('origin', 'like', 'stock.order_point,%'),
                 ])