[BUG] - The registration center response exception cause nullPointerException.

### Steps to Reproduce

java.lang.NullPointerException: null
at org.apache.servicecomb.service.center.client.ServiceCenterRegistration$SendHeartBeatTask.execute(ServiceCenterRegistration.java:294) ~[service-center-client-2.8.26.jar:2.8.26]
at org.apache.servicecomb.http.client.task.AbstractTask.lambda$startTask$2(AbstractTask.java:94) ~[http-client-common-2.8.26.jar:2.8.26]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[na:1.8.0_372]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[na:1.8.0_372]
at java.lang.Thread.run(Thread.java:750) ~[na:1.8.0_372]

### Expected Behavior

_No response_

### Servicecomb Version

_No response_

### Additional Context

_No response_

Start at ServiceCenterRegistration$SendHeartBeatTask.execute in service-center-client, especially line 294 from the stack trace, and trace the response handling that leads to the NullPointerException. Reproduce the heartbeat failure if possible and identify the missing context, since the issue gives no expected behavior or test location. Done means the reported registration-center response path no longer throws this NullPointerException and its failure cause remains actionable.

Written by the indexing model from the issue text.