Fill tax rule pattern with default empty countries

A rule line for a country is not expected to be used when the country is
unknown.

Closes #13169
(grafted from cea3c89a740248a84ce48a906f1b1c33ba0fac5e)

--HG--
branch : 7.0

modules/account_tax_rule_country/account.py

@@ -19,6 +19,8 @@ class TaxRule(metaclass=PoolMeta):
                 subdivision = subdivision.parent
 
         pattern = pattern.copy()
+        pattern.setdefault('from_country')
+        pattern.setdefault('to_country')
         for name in ['from_subdivision', 'to_subdivision']:
             subdivision = pattern.pop(name, None)
             if not subdivision: