全局变量g_tAuctionString不再有效导致的bug

[hycinth22]

问题表现：
当关闭“实时竞拍倒计时”功能后，将导致交易行显示<600s物品时，访问g_tAuctionString而直接报错，进而导致搜索页面为空。

游戏日志：

ERROR [KGUI] KGUI lua call failed [...JX3\bin\zhcn_hd\interface\AH\AH_Helper\ah_helper.lua:0: attempt to index global 'g_tAuctionString' (a nil value)]
stack traceback:
[C]: ?
...JX3\bin\zhcn_hd\interface\AH\AH_Helper\ah_helper.lua: in function 'FormatAuctionTime'
...JX3\bin\zhcn_hd\interface\AH\AH_Helper\ah_helper.lua: in function 'SetSaleInfo'
...JX3\bin\zhcn_hd\interface\AH\AH_Helper\ah_helper.lua: in function 'UpdateItemList'
\ui\config\default\AuctionPanel.lua: in function <\ui\config\default\AuctionPanel.lua:167>

问题代码：
https://github.com/phinexdaz/ah/blob/e4412053cd774196999bab5393d8be3822e13e08/AH_Helper/AH_Helper.lua#L524