[pulsar-client] NPE while call getLastMessageId

[murong00]

Describe the bug
Meet NPE while call getLastMessageId as below:

[root@xxx apache-pulsar-2.6.0-SNAPSHOT]# bin/pulsar-admin topics last-message-id persistent://public/default/test
java.lang.RuntimeException: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.apache.pulsar.client.impl.BatchMessageIdImpl["acker"]->org.apache.pulsar.client.impl.BatchMessageAcker["outstandingAcks"])
        at org.apache.pulsar.admin.cli.CliCommand.print(CliCommand.java:182)
        at org.apache.pulsar.admin.cli.CmdTopics$GetLastMessageId.run(CmdTopics.java:773)
        at org.apache.pulsar.admin.cli.CmdBase.run(CmdBase.java:71)
        at org.apache.pulsar.admin.cli.PulsarAdminTool.run(PulsarAdminTool.java:221)
        at org.apache.pulsar.admin.cli.PulsarAdminTool.main(PulsarAdminTool.java:263)
Caused by: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.apache.pulsar.client.impl.BatchMessageIdImpl["acker"]->org.apache.pulsar.client.impl.BatchMessageAcker["outstandingAcks"])
        at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:394)
        at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:353)
        at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316)
        at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:730)
        at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166)
        at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
        at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722)
        at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166)
        at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
        at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
        at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1433)
        at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1135)
        at com.fasterxml.jackson.databind.ObjectWriter.writeValueAsString(ObjectWriter.java:1005)
        at org.apache.pulsar.admin.cli.CliCommand.print(CliCommand.java:180)
        ... 4 more
Caused by: java.lang.NullPointerException
        at org.apache.pulsar.client.impl.BatchMessageAcker.getOutstandingAcks(BatchMessageAcker.java:69)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:688)
        at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722)
        ... 14 more

The reason is that BatchMessageIdImpl use a default BatchMessageAcker with null value.

Expected behavior
getLastMessageId should be successful like below:

[root@xxx apache-pulsar-2.6.0-SNAPSHOT]# bin/pulsar-admin topics last-message-id persistent://public/default/test
{
  "ledgerId" : 5,
  "entryId" : 0,
  "partitionIndex" : -1,
  "batchIndex" : 9,
  "acker" : {
    "batchSize" : 0,
    "prevBatchCumulativelyAcked" : false,
    "outstandingAcks" : 0
  },
  "outstandingAcksInSameBatch" : 0,
  "batchSize" : 0
}