[lua]: use not to check whether the field exists (sonic-net#492)

Signed-off-by: Sihui Han <sihan@microsoft.com>

orchagent/pfc_restore.lua

@@ -24,7 +24,7 @@ for i = n, 1, -1 do
     if not big_red_switch_mode and pfc_wd_status ~= 'operational'  and pfc_wd_action ~= 'alert' and restoration_time and restoration_time ~= '' then
         restoration_time = tonumber(restoration_time)
         local time_left = redis.call('HGET', counters_table_name .. ':' .. KEYS[i], 'PFC_WD_RESTORATION_TIME_LEFT')
-        if time_left == nil then
+        if not time_left then
             time_left = restoration_time
         else
             time_left = tonumber(time_left)