Do not convert empty origin to string to check against sale opportunity value

c5291471fd83 added a comparison with the value but it fails for None value.

Closes #13832
(grafted from 236f01bc6fcb26e482051884eb84a80fba54403f)

--HG--
branch : 7.0

modules/sale_opportunity/sale.py

@@ -38,7 +38,7 @@ class Sale(metaclass=PoolMeta):
         for sales, values in zip(actions, actions):
             if 'origin' in values:
                 origin = values['origin']
-                if not isinstance(origin, str):
+                if origin and not isinstance(origin, str):
                     origin = '%s,%s' % tuple(origin)
                 for sale in sales:
                     if (isinstance(sale.origin, Opportunity)