Only delete draft requests when generating new ones

We can not rely on purchase_line as purchase_request_quotation adds new
states before the request is purchased that should not be deleted

issue11479
review427161003
(grafted from f4bea63d827ac809f57980bb335031b570017d3d)

--HG--
branch : 6.0

purchase_request.py

@@ -132,7 +132,7 @@ class PurchaseRequest(metaclass=PoolMeta):
         # delete purchase requests without a purchase line
         products = set(products)
         reqs = cls.search([
-                ('purchase_line', '=', None),
+                ('state', '=', 'draft'),
                 ('origin', 'like', 'stock.order_point,%'),
                 ])
         reqs = [r for r in reqs