fix IsBilling not turn off

EVCB_OCPP.WSServer/Service/DbService/WebDbService.cs

@@ -98,6 +98,7 @@ public class WebDbService
                 var result = await conn.QueryFirstOrDefaultAsync<StationFee>(displayPricestrSql, parameters);
                 if (result == default)
                 {
+                    client.IsBilling = false;
                     return string.Empty;
                 }
                 var stationPrice = result;//.First();