Flaky-test: NegativeAcksTest.testNegativeAcks

[example failure](https://github.com/apache/pulsar/runs/7644153917?check_suite_focus=true)

```
Error: Tests run: 128, Failures: 4, Errors: 0, Skipped: 121, Time elapsed: 64.753 s <<< FAILURE! - in org.apache.pulsar.client.impl.NegativeAcksTest
Error: testNegativeAcks(org.apache.pulsar.client.impl.NegativeAcksTest) Time elapsed: 0.869 s <<< FAILURE!
java.lang.AssertionError: expected [null] but found [org.apache.pulsar.client.impl.TopicMessageImpl@12da2698]
at org.testng.Assert.fail(Assert.java:99)
at org.testng.Assert.failNotSame(Assert.java:1033)
at org.testng.Assert.assertNull(Assert.java:965)
at org.testng.Assert.assertNull(Assert.java:953)
at org.apache.pulsar.client.impl.NegativeAcksTest.testNegativeAcks(NegativeAcksTest.java:160)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
at org.testng.internal.InvokeMethodRunnable.runOne(InvokeMethodRunnable.java:45)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:73)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:11)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:[1136](https://github.com/apache/pulsar/runs/7644153917?check_suite_focus=true#step:10:1137))
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
```

Start with org.apache.pulsar.client.impl.NegativeAcksTest.testNegativeAcks at line 160 and review the linked CI failure. Reproduce the test and determine why it returns a TopicMessageImpl where null is expected; done means the test reliably passes without masking the negative-ack behavior.

Written by the indexing model from the issue text.