Fix #2051: don't try to sort None
Fix #2051: don't try to sort None

I found the point in the test harnesses where None was introduced as a value
for `TrackInfo.index`. Just using zero works fine.

madmouser1/beets,beetbox/beets,shamangeorge/beets,jackwilsdon/beets,ibmibmibm/beets,mosesfistos1/beetbox,MyTunesFreeMusic/privacy-policy,lengtche/beets,jackwilsdon/beets,diego-plan9/beets,sampsyo/beets,beetbox/beets,MyTunesFreeMusic/privacy-policy,madmouser1/beets,Kraymer/beets,ibmibmibm/beets,mosesfistos1/beetbox,diego-plan9/beets,beetbox/beets,mosesfistos1/beetbox,xsteadfastx/beets,lengtche/beets,madmouser1/beets,jcoady9/beets,jcoady9/beets,artemutin/beets,beetbox/beets,sampsyo/beets,jackwilsdon/beets,ibmibmibm/beets,xsteadfastx/beets,shamangeorge/beets,diego-plan9/beets,artemutin/beets,SusannaMaria/beets,shamangeorge/beets,pkess/beets,SusannaMaria/beets,pkess/beets,SusannaMaria/beets,Kraymer/beets,jcoady9/beets,ibmibmibm/beets,artemutin/beets,artemutin/beets,shamangeorge/beets,mosesfistos1/beetbox,SusannaMaria/beets,lengtche/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,Kraymer/beets,lengtche/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,jcoady9/beets,xsteadfastx/beets,sampsyo/beets,xsteadfastx/beets,diego-plan9/beets,Kraymer/beets,pkess/beets,jackwilsdon/beets,madmouser1/beets