fix: 修复 AccountOnChainMonitorService 中 JsonNull 导致的崩溃

- 增加 receiptRpcResponse.result.isJsonNull 检查
- 防止在链上查询返回 JsonNull 时调用 asJsonObject() 抛出 IllegalStateException
- 优化空值处理逻辑，确保程序安全退出

backend/src/main/kotlin/com/wrbug/polymarketbot/service/copytrading/monitor/AccountOnChainMonitorService.kt

@@ -114,10 +114,10 @@ class AccountOnChainMonitorService(
             }
             
             val receiptRpcResponse = receiptResponse.body()!!
-            if (receiptRpcResponse.error != null || receiptRpcResponse.result == null) {
+            if (receiptRpcResponse.error != null || receiptRpcResponse.result == null || receiptRpcResponse.result.isJsonNull) {
                 return
             }
-            
+
             // 使用 Gson 解析 receipt JSON
             val receiptJson = receiptRpcResponse.result.asJsonObject