[fix] util.isEmptyDir() crashes on non-existent dir
[fix] util.isEmptyDir() crashes on non-existent dir

lfs.dir will crash rather than return nil if directory doesn't exist

Proper fix for 9f5e44670183d046054f87b4820bf5d1b35b3b12 which is nothing but a workaround. However, I do think creating more of those data dirs automatically is more user-friendly because otherwise Android users will have to look it up or guess.

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