ClustersServiceImpl.getClustersList err:NullPointerException

Turn on JWT authentication

```
java.lang.NullPointerException: null
at org.apache.pulsar.manager.service.impl.ClustersServiceImpl.getClustersList(ClustersServiceImpl.java:62) ~[pulsar-manager.jar:na]
at org.apache.pulsar.manager.service.impl.BrokerStatsServiceImpl.scheduleCollectStats(BrokerStatsServiceImpl.java:128) ~[pulsar-manager.jar:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_212]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_212]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_212]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_212]
at org.springframework.scheduling.support.ScheduledMethodRunnable.run(ScheduledMethodRunnable.java:65) ~[spring-context-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at org.springframework.scheduling.support.DelegatingErrorHandlingRunnable.run(DelegatingErrorHandlingRunnable.java:54) ~[spring-context-5.0.6.RELEASE.jar:5.0.6.RELEASE]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_212]
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) [na:1.8.0_212]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) [na:1.8.0_212]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) [na:1.8.0_212]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_212]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_212]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_212]
```

2020-07-16 11:38:58.268 DEBUG 54 --- [SimpleHostRoutingFilter.connectionManagerTimer] h.i.c.PoolingHttpClientConnectionManager : Closing expired connections
2020-07-16 11:39:03.268 DEBUG 54 --- [SimpleHostRoutingFilter.connectionManagerTimer] h.i.c.PoolingHttpClientConnectionManager : Closing expired connections