Some transforms, such as the EVR mixer do not implement GetInputAvailableType and may not have any current media type set, though they would accept upstream media type right away.
-- v2: mf: Only enumerate types for decoders / converter if down type is missing. mf: Only allow converter connection if decoder didn't accept down type. mf: Allow resolving topology with missing downstream current type. mf/tests: Add more topology loader converter and enumeration tests. mf/tests: Add some MP3 decoder transform tests. mf/tests: Add some topology loader tests with missing output type.