### Motivation

Broker throws NPE when pulsar-admin tries to fetch stats-internal for topic with reader.

```
Caused by: java.lang.NullPointerException
	at org.apache.bookkeeper.mledger.impl.ManagedCursorImpl.getProperties(ManagedCursorImpl.java:234) ~[classes/:?]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.lambda$getInternalStats$48(PersistentTopic.java:1461) ~[classes/:?]
	at java.lang.Iterable.forEach(Iterable.java:75) ~[?:1.8.0_92]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.getInternalStats(PersistentTopic.java:1446) ~[classes/:?]
	at org.apache.pulsar.broker.admin.impl.PersistentTopicsBase.internalGetInternalStats(PersistentTopicsBase.java:621) ~[classes/:?]
	at org.apache.pulsar.broker.admin.v2.PersistentTopics.getInternalStats(PersistentTopics.java:430) ~[classes/:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_92]
```
(cherry picked from commit b306552f)