Fix too broad truthness test (fix #501)

This caused TlTracks with tlid=0 to be sent to HTTP clients without the tlid
field.

docs/changelog.rst

@@ -64,6 +64,12 @@ v0.15.0 (UNRELEASED)
   The methods are still not implemented, but now the commands are accepted as
   valid.
 
+**HTTP frontend**
+
+- Fix too broad truthness test that caused :class:`~mopidy.models.TlTrack`
+  objects with ``tlid`` set to ``0`` to be sent to the HTTP client without the
+  ``tlid`` field. (Fixes: :issue:`501`)
+
 **Extension support**
 
 - :class:`~mopidy.config.Secret` is now deserialized to unicode strings instead

mopidy/models.py

@@ -90,7 +90,7 @@ class ImmutableObject(object):
                     for v in value]
             elif isinstance(value, ImmutableObject):
                 value = value.serialize()
-            if value:
+            if not (isinstance(value, list) and len(value) == 0):
                 data[public_key] = value
         return data
 

tests/models_test.py

@@ -95,6 +95,11 @@ class ArtistTest(unittest.TestCase):
             {'__model__': 'Artist', 'uri': 'uri', 'name': 'name'},
             Artist(uri='uri', name='name').serialize())
 
+    def test_serialize_falsy_values(self):
+        self.assertDictEqual(
+            {'__model__': 'Artist', 'uri': '', 'name': None},
+            Artist(uri='', name=None).serialize())
+
     def test_to_json_and_back(self):
         artist1 = Artist(uri='uri', name='name')
         serialized = json.dumps(artist1, cls=ModelJSONEncoder)