HDFS-17133. TestFsDatasetImpl missing null check when cleaning up

[teamconfx]

Description of PR

https://issues.apache.org/jira/browse/HDFS-17133
This PR adds a check that cluster is not null before cleaning up and shutting down the cluster.

How was this patch tested?

1. set dfs.namenode.quota.init-threads=1468568631
2. run org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.TestFsDatasetImpl#testMoveBlockFailure
   The test fails with the IllegalArgumentException rather than the NullPointerException.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 31s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
+1 💚	mvninstall	34m 33s		trunk passed
+1 💚	compile	0m 54s		trunk passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	compile	0m 50s		trunk passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	checkstyle	0m 45s		trunk passed
+1 💚	mvnsite	0m 56s		trunk passed
+1 💚	javadoc	0m 52s		trunk passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	1m 21s		trunk passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	spotbugs	2m 5s		trunk passed
+1 💚	shadedclient	25m 16s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 49s		the patch passed
+1 💚	compile	0m 50s		the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javac	0m 50s		the patch passed
+1 💚	compile	0m 45s		the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	javac	0m 45s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 38s		the patch passed
+1 💚	mvnsite	0m 46s		the patch passed
+1 💚	javadoc	0m 41s		the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	1m 16s		the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	spotbugs	2m 10s		the patch passed
+1 💚	shadedclient	26m 13s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	215m 32s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 40s		The patch does not generate ASF License warnings.
		320m 35s

Reason	Tests
Failed junit tests	hadoop.hdfs.server.namenode.snapshot.TestSnapshotDiffReport
	hadoop.hdfs.server.namenode.snapshot.TestRenameWithSnapshots
	hadoop.hdfs.server.namenode.snapshot.TestSnapshottableDirListing

Subsystem	Report/Notes
Docker	ClientAPI=1.43 ServerAPI=1.43 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6079/1/artifact/out/Dockerfile
GITHUB PR	#6079
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 16458bd9b07f 4.15.0-213-generic #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 7dd1933
Default Java	Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6079/1/testReport/
Max. process+thread count	3373 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6079/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[ayushtkn]

LGTM