Fix JSON encoding: force empty tables to encode as arrays

Lua's json.encode produces {} for empty tables instead of [].
Use json.util.InitArray to mark tables as arrays so they encode
correctly as [] for authors, bookmarks, highlights, notes, and books.

BookhoardAPI.lua

@@ -1,4 +1,5 @@
 local json = require("json")
+local json_util = require("json/util")
 local logger = require("logger")
 local ltn12 = require("ltn12")
 local socketutil = require("socketutil")
@@ -118,8 +119,9 @@ function BookhoardAPI:checkRegistrationStatus(registration_id)
 end
 
 function BookhoardAPI:syncProgress(book_data, sync_mode)
+    local books = json_util.InitArray({ book_data })
     return self:_request("POST", "/api/sync/koreader/progress", {
-        books = { book_data },
+        books = books,
         sync_mode = sync_mode or "immediate",
     }, PROGRESS_TIMEOUTS)
 end