Add null check to workaround NPE in unit tests with Mockito/PowerMock

[lhotari]

• Fixes Flaky-test: Spy creation sporadically fails in some tests and causes Mockito misuse warning #13620

Motivation

Workaround for a NPE issue that happens with Mockito/PowerMock:

org.mockito.exceptions.base.MockitoException: Unable to create mock instance of type 'ServerCnx'
	at org.apache.pulsar.broker.BrokerTestUtil.spyWithClassAndConstructorArgs(BrokerTestUtil.java:43)
	at 
	... 
	... 
	... 
Caused by: java.lang.reflect.InvocationTargetException
	at org.mockito.internal.creation.bytebuddy.ByteBuddyMockMaker.createMock(ByteBuddyMockMaker.java:43)
	at org.powermock.api.mockito.mockmaker.PowerMockMaker.createMock(PowerMockMaker.java:41)
	at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:53)
	at org.mockito.internal.MockitoCore.mock(MockitoCore.java:84)
	at org.mockito.Mockito.mock(Mockito.java:1964)
	at org.apache.pulsar.broker.BrokerTestUtil.spyWithClassAndConstructorArgs(BrokerTestUtil.java:43)
	at org.apache.pulsar.broker.service.PersistentTopicTest.setup(PersistentTopicTest.java:211)
	at org.apache.pulsar.broker.service.persistent.PersistentTopicStreamingDispatcherTest.setup(PersistentTopicStreamingDispatcherTest.java:34)
	at jdk.internal.reflect.GeneratedMethodAccessor332.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	... 34 more
Caused by: java.lang.NullPointerException
	at org.apache.pulsar.broker.service.ServerCnx.<init>(ServerCnx.java:234)
	at org.apache.pulsar.broker.service.ServerCnx.<init>(ServerCnx.java:230)
	at org.apache.pulsar.broker.service.ServerCnx$MockitoMock$985617261.<init>(Unknown Source)
	... 55 more

Modification

Add a null check to prevent the NPE.

[github-actions]

@lhotari:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@lhotari:Thanks for providing doc info!

[michaeljmarshall]

I believe this would also fix #13808.

[michaeljmarshall]

LGTM. If/when we remove the mocking that is responsible for these flaky tests, it'd be good to revert this change.