Fix handling of null MX records and add test for behavior

octodns/record/__init__.py

@@ -1090,8 +1090,9 @@ class MxValue(EqualityTupleMixin):
                 reasons.append(f'invalid preference "{value["preference"]}"')
             exchange = None
             try:
-                exchange = value.get('exchange', None) or value['value']
-                if not FQDN(str(exchange), allow_underscores=True).is_valid:
+                exchange = str(value.get('exchange', None) or value['value'])
+                if exchange != '.' and \
+                        not FQDN(exchange, allow_underscores=True).is_valid:
                     reasons.append(f'Invalid MX exchange "{exchange}" is not '
                                    'a valid FQDN.')
                 elif not exchange.endswith('.'):

tests/test_octodns_record.py

@@ -2717,6 +2717,17 @@ class TestRecordValidation(TestCase):
         self.assertEqual(['Invalid MX exchange "100 foo.bar.com." is not a '
                           'valid FQDN.'], ctx.exception.reasons)
 
+        # exchange can be a single `.`
+        record = Record.new(self.zone, '', {
+            'type': 'MX',
+            'ttl': 600,
+            'value': {
+                'preference': 0,
+                'exchange': '.'
+            }
+        })
+        self.assertEqual('.', record.values[0].exchange)
+
     def test_NXPTR(self):
         # doesn't blow up
         Record.new(self.zone, '', {