Convert empty string to None for match value

It is not possible from the clients to force to store None value instead of
empty string.

Closes #13890

trytond/trytond/model/match.py

@@ -11,6 +11,8 @@ class MatchMixin(object):
         and matching value as value'''
         for field_name, pattern_value in pattern.items():
             value = getattr(self, field_name)
+            if value == '':
+                value = None
             if not match_none and value is None:
                 continue
             field = self._fields[field_name]

trytond/trytond/tests/test_model_match.py

@@ -35,7 +35,9 @@ class ModelMatchMixinTestCase(TestCase):
                 (Model(f=(fields.Char("F"), 'foo')), {'f': 'bar'}, False),
                 (Model(f=(fields.Char("F"), 'foo')), {'f': None}, False),
                 (Model(f=(fields.Char("F"), None)), {'f': 'foo'}, True),
+                (Model(f=(fields.Char("F"), '')), {'f': 'foo'}, True),
                 (Model(f=(fields.Char("F"), None)), {'f': None}, True),
+                (Model(f=(fields.Char("F"), '')), {'f': None}, True),
                 ]:
             with self.subTest(record=record, pattern=pattern):
                 self.assertEqual(record.match(pattern), result)
@@ -47,7 +49,9 @@ class ModelMatchMixinTestCase(TestCase):
                 (Model(f=(fields.Char("F"), 'foo')), {'f': 'bar'}, False),
                 (Model(f=(fields.Char("F"), 'foo')), {'f': None}, False),
                 (Model(f=(fields.Char("F"), None)), {'f': 'foo'}, False),
+                (Model(f=(fields.Char("F"), '')), {'f': 'foo'}, False),
                 (Model(f=(fields.Char("F"), None)), {'f': None}, True),
+                (Model(f=(fields.Char("F"), '')), {'f': None}, True),
                 ]:
             with self.subTest(record=record, pattern=pattern):
                 self.assertEqual(