[fix] Minor kodev shellscript issues (#4302)
[fix] Minor kodev shellscript issues (#4302)

Use `-z` instead of `! -n`.

See https://github.com/koalaman/shellcheck/wiki/SC2236
mwoz123/koreader,koreader/koreader,NiLuJe/koreader,poire-z/koreader,mihailim/koreader,Markismus/koreader,Frenzie/koreader,Hzj-jie/koreader,pazos/koreader,NiLuJe/koreader,houqp/koreader,poire-z/koreader,koreader/koreader,Frenzie/koreader