Multi-representation video-only OR audio-only streams throw TypeError

Created by: davemevans

#1580 (closed) and #1586 (closed) were both fixed by #1581 but the fix provided only works for streams with a single representation.

http://rdmedia.bbc.co.uk/dash/ondemand/bbb/2/client_manifest-audio.mpd, for example, is audio-only, with multiple audio representations. Attempting to play this throws Uncaught TypeError: Cannot read property 'length' of null. This is because createMenu is called (because audio.length is > 1) and still assumes that info.video exists.

I think the answer is to ensure that these attributes always exist but with length 0.