config: Lists need to handle missing values

mopidy/config/types.py

@@ -185,6 +185,8 @@ class List(ConfigValue):
         return tuple(values)
 
     def serialize(self, value, display=False):
+        if not value:
+            return b''
         return b'\n  ' + b'\n  '.join(encode(v) for v in value if v)
 
 

tests/config/test_types.py

@@ -268,6 +268,12 @@ class ListTest(unittest.TestCase):
         self.assertIsInstance(result, bytes)
         self.assertRegexpMatches(result, r'foo\n\s*bar\n\s*baz')
 
+    def test_serialize_none(self):
+        value = types.List()
+        result = value.serialize(None)
+        self.assertIsInstance(result, bytes)
+        self.assertEqual(result, '')
+
 
 class LogLevelTest(unittest.TestCase):
     levels = {'critical': logging.CRITICAL,