fix(ReaderFooter:getDataFromStatistics): return na if book has no stats history
fix(ReaderFooter:getDataFromStatistics): return na if book has no stats history

mihailim/koreader,koreader/koreader,NiLuJe/koreader,Frenzie/koreader,chihyang/koreader,apletnev/koreader,poire-z/koreader,NiLuJe/koreader,pazos/koreader,Markismus/koreader,lgeek/koreader,poire-z/koreader,koreader/koreader,mwoz123/koreader,Hzj-jie/koreader,NickSavage/koreader,robert00s/koreader,Frenzie/koreader,houqp/koreader,frankyifei/koreader