gst1: Fail if trying to create buffers without audio

Which causes lots of failed assertion messages from GStreamer

mopidy/audio/utils.py

@@ -28,6 +28,9 @@ def create_buffer(data, capabilites=None, timestamp=None, duration=None):
     .. versionchanged:: 1.2
         ``capabilites`` argument is no longer in use
     """
+    if not data:
+        raise ValueError(
+            'Cannot create buffer without data: length=%d' % len(data))
     buffer_ = Gst.Buffer.new_wrapped(data)
     if timestamp is not None:
         buffer_.pts = timestamp

tests/audio/test_utils.py

@@ -23,6 +23,12 @@ class TestCreateBuffer(object):
         assert buf.duration == 1000000
         assert buf.get_size() == len(b'123')
 
+    def test_fails_if_data_has_zero_length(self):
+        with pytest.raises(ValueError) as excinfo:
+            utils.create_buffer(b'', timestamp=0, duration=1000000)
+
+        assert 'Cannot create buffer without data' in str(excinfo.value)
+
 
 # TODO: keep ids without name?
 # TODO: current test is trying to test everything at once with a complete tags