Check position against null, not zero
Check position against null, not zero

a8ac3142 had a bug in which a switch at position zero would not result
in clearing anything.  Positions may validly be zero, but never null.

Change-Id: I95a607df7bd6a41cf73f78122c3dcdda425fb145

shaka-project/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,brightcove/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,Ross-cz/shaka-player,tvoli/shaka-player,TobbeEdgeware/shaka-player,vimond/shaka-player,russitto/shaka-player,TheModMaker/shaka-player,brightcove/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,vimond/shaka-player,indiereign/shaka-player,indiereign/shaka-player,brightcove/shaka-player,Ross-cz/shaka-player,TobbeEdgeware/shaka-player,russitto/shaka-player,russitto/shaka-player,shaka-project/shaka-player,TobbeEdgeware/shaka-player,vimond/shaka-player,tvoli/shaka-player,Ross-cz/shaka-player,indiereign/shaka-player,TheModMaker/shaka-player