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.

main.lua

@@ -11,6 +11,7 @@ local SpinWidget = require("ui/widget/spinwidget")
 local UIManager = require("ui/uimanager")
 local WidgetContainer = require("ui/widget/container/widgetcontainer")
 local json = require("json")
+local json_util = require("json/util")
 local logger = require("logger")
 local sha2 = require("ffi/sha2")
 local time = require("ui/time")
@@ -670,6 +671,7 @@ function Bookhoard:collectBookData()
     if props.authors then
         authors = { props.authors }
     end
+    json_util.InitArray(authors)
 
     local file_sha256 = self:getFileSHA256()
     local book_uuid = self:getBookhoardUUID()
@@ -699,9 +701,9 @@ function Bookhoard:collectBookData()
 end
 
 function Bookhoard:collectAnnotations()
-    local bookmarks = {}
-    local highlights = {}
-    local notes = {}
+    local bookmarks = json_util.InitArray({})
+    local highlights = json_util.InitArray({})
+    local notes = json_util.InitArray({})
 
     if not self.ui.bookmark then
         return bookmarks, highlights, notes