Test for create_request should allow zero quantity

Since changeset c12f565887e7, it is allowed to have a quantity of zero for the
purchase request. So the test for manual creation should not catch this case.

issue5328
review21701002

purchase_request.py

@@ -539,7 +539,7 @@ class PurchaseRequest(ModelSQL, ModelView):
     def create(cls, vlist):
         for vals in vlist:
             for field_name in ('product', 'quantity', 'uom', 'company'):
-                if not vals.get(field_name):
+                if vals.get(field_name) is None:
                     cls.raise_user_error('create_request')
         return super(PurchaseRequest, cls).create(vlist)