[fix](fe ut) Fix borrow oject throw npe (apache#27072) (apache#27207)

occasional failure of fe ut, borrowObject throw npe
```
get agent task request. type: CREATE, signature: 10008, fe addr: null
java.lang.NullPointerException
	at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)
	at org.apache.commons.pool2.impl.GenericKeyedObjectPool.register(GenericKeyedObjectPool.java:1079)
	at org.apache.commons.pool2.impl.GenericKeyedObjectPool.borrowObject(GenericKeyedObjectPool.java:347)
get agent task request. type: CREATE, signature: 10012, fe addr: TNetworkAddress(hostname:127.0.0.1, port:56072)
	at org.apache.commons.pool2.impl.GenericKeyedObjectPool.borrowObject(GenericKeyedObjectPool.java:277)
	at org.apache.doris.common.GenericPool.borrowObject(GenericPool.java:99)
	at org.apache.doris.utframe.MockedBackendFactory$DefaultBeThriftServiceImpl$1.run(MockedBackendFactory.java:219)
	at java.lang.Thread.run(Thread.java:750)
```

fe/fe-core/src/test/java/org/apache/doris/utframe/MockedBackendFactory.java

@@ -167,8 +167,12 @@ public void run() {
  FrontendService.Client client = null;
  TNetworkAddress address = null;
  try {
- address = backend.getFeAddress();
+ // ATTR: backend.getFeAddress must after taskQueue.take, because fe addr thread race
  TAgentTaskRequest request = taskQueue.take();
+ address = backend.getFeAddress();
+ if (address == null) {
+ System.out.println("fe addr thread race, please check it");
+ }
  System.out.println(
  "get agent task request. type: " + request.getTaskType() + ", signature: "
  + request.getSignature() + ", fe addr: " + address);