test: fix npe issue introduced by #20933

zookeeper 3.9.1 add a check to print a log.
```java
                if (addr.getPort() == 0) {
                    LOG.info("bound to port {}", this.ss.getLocalAddress());
                }
```

managed-ledger/src/test/java/org/apache/bookkeeper/test/ZooKeeperUtil.java

@@ -25,6 +25,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.util.concurrent.CountDownLatch;
 
@@ -54,6 +55,9 @@ public class ZooKeeperUtil {
     private String connectString;
 
     public ZooKeeperUtil() {
+        String loopbackIPAddr = InetAddress.getLoopbackAddress().getHostAddress();
+        zkaddr = new InetSocketAddress(loopbackIPAddr, 0);
+        connectString = loopbackIPAddr + ":" + zooKeeperPort;
     }
 
     public ZooKeeper getZooKeeperClient() {