Fix Pocketbook crashes when filename is nil (#8823)

Function `SetSubtaskInfo` takes only chars as a valid book. Since
filename can be `nil` (not a char) the crash would happen. This
Change avoids the crash by using a char type nil value. See:

`int SetSubtaskInfo(int task, int subtask, const char *name, const char *book);`

Fixes #8053
Fixes #8392 too
Fixes #8743 last issue

frontend/device/pocketbook/device.lua

@@ -5,6 +5,7 @@ local C = ffi.C
 local inkview = ffi.load("inkview")
 local band = require("bit").band
 local util = require("util")
+local _ = require("gettext")
 
 require("ffi/posix_h")
 require("ffi/linux_input_h")
@@ -268,7 +269,7 @@ function PocketBook:notifyBookState(title, document)
         fo:write(fn)
         fo:close()
     end
-    inkview.SetSubtaskInfo(inkview.GetCurrentTask(), 0, title and (title .. " - koreader") or "koreader", fn)
+    inkview.SetSubtaskInfo(inkview.GetCurrentTask(), 0, title and (title .. " - koreader") or "koreader", fn or _("N/A"))
 end
 
 function PocketBook:setDateTime(year, month, day, hour, min, sec)