[Fix] fix custom monitoring bulletin NullPointerException

[Duansg]

What's changed?

In the Bulletin list, if there is a deleted Monitor, the page and system will throw an exception. Although I can manually delete it, I expect the page to function normally

2025-06-10 09:42:39.952 [http-nio-1157-exec-1] ERROR org.apache.hertzbeat.manager.support.GlobalExceptionHandler Line:252 - [monitor]-[unknown error happen]-Cannot invoke "org.apache.hertzbeat.common.entity.manager.Monitor.getName()" because "monitor" is null  
java.lang.NullPointerException: Cannot invoke "org.apache.hertzbeat.common.entity.manager.Monitor.getName()" because "monitor" is null  
    at org.apache.hertzbeat.manager.service.impl.BulletinServiceImpl.buildBulletinMetricsData(BulletinServiceImpl.java:125)    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.base/java.lang.reflect.Method.invoke(Method.java:568)    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:359) 

Added a null check validation; if Monitor is null, subsequent logic will not be executed

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[LiuTianyou]

LGTM！👍🏻