Output.get_position() should return None if position is unknown

mopidy/outputs/base.py

@@ -67,7 +67,7 @@ class BaseOutput(object):
 
         *MUST be implemented by subclass.*
 
-        :rtype: int
+        :rtype: int or :class:`None` if unknown
         """
         raise NotImplementedError
 

mopidy/outputs/dummy.py

@@ -32,7 +32,7 @@ class DummyOutput(BaseOutput):
     end_of_data_stream_called = False
 
     #: For testing. Contains the current position.
-    position = 0
+    position = None
 
     #: For testing. Contains the current state.
     state = 'NULL'