[Bug] 使用@Cacheable缓存外部接口结果，当配置了缓存为空继续执行请求接口，回放请求报NullPointerException

### Search before asking

- [x] I have searched the existing [issues](https://github.com/arextest/arex-agent-java/issues) before asking.

### AREX Test Service

AREX Java Agent (arextest/arex-agent-java)

### Current Behavior

当使用@Cacheable缓存外部接口的结果，配置unless = "#result == null" 缓存不存在继续执行外部请求时，回放该请求会报NPE
**示例代码如下：**

`@Cacheable(value = "TEST", key = "#id", unless = "#result == null") `

`@Override public ProductDTO getProduct(final Integer id) { `

`final DataResponse dataResponse = client.getProductById(id); `

`return dataResponse.getData(); `

`}`

**Mock的数据如下：**

- 只有getProduct(final Integer id) 这个方法的mock，client.getProductById(id)并没有看到mock信息

**报错信息：**
`java.lang.NullPointerException: `

`null at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:158) at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:78) at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103) at `

`com.sun.proxy.$Proxy239.getProductById(Unknown Source)`

**本地debug后初步分析：**

- 在回放流量的时候，会继续执行client.getProductById方法但是因为该方法并没有mock数据导致报了NPE

### Expected Behavior

执行client.getProductById不要报NPE

### Steps To Reproduce

**组件版本**

- spring-cloud-starter-openfeign 2.1.0
- httpclient 4.5.12

### Anything else

无

### Are you willing to submit a pull request to fix on your own?

- [ ] Yes I am willing to submit a pull request on my own!

Start with the replay path for a @Cacheable method using unless = "#result == null" and trace the call to client.getProductById(id), which lacks mock data in the report. Reproduce with spring-cloud-starter-openfeign 2.1.0 and httpclient 4.5.12; done means replaying this case does not raise the reported NullPointerException.

Written by the indexing model from the issue text.