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

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

Proper fix for 9f5e446701 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.

frontend/util.lua

@@ -288,7 +288,10 @@ end
 ---- @treturn bool
 function util.isEmptyDir(path)
     local lfs = require("libs/libkoreader-lfs")
-    for filename in lfs.dir(path) do
+    -- lfs.dir will crash rather than return nil if directory doesn't exist O_o
+    local ok, iter, dir_obj = pcall(lfs.dir, path)
+    if not ok then return end
+    for filename in iter, dir_obj do
         if filename ~= '.' and filename ~= '..' then
             return false
         end