TEZ-4321: Do Not Wrap Singleton Collections

[belugabehr]

Singleton Collections are by their design immutable. There is no need to wrap them in an unmodifiableCollection.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	12m 49s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
+1 💚	test4tests	0m 1s	The patch appears to include 1 new or modified test files.
		_ master Compile Tests _
+1 💚	mvninstall	13m 40s	master passed
+1 💚	compile	0m 45s	master passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	compile	0m 37s	master passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	checkstyle	1m 15s	master passed
+1 💚	javadoc	0m 44s	master passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javadoc	0m 34s	master passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+0 🆗	spotbugs	1m 41s	Used deprecated FindBugs config; considering switching to SpotBugs.
+1 💚	findbugs	1m 39s	master passed
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 25s	the patch passed
+1 💚	compile	0m 29s	the patch passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javac	0m 29s	the patch passed
+1 💚	compile	0m 25s	the patch passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	javac	0m 25s	the patch passed
-0 ⚠️	checkstyle	0m 18s	tez-dag: The patch generated 3 new + 25 unchanged - 3 fixed = 28 total (was 28)
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	javadoc	0m 22s	the patch passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javadoc	0m 21s	the patch passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	findbugs	1m 10s	the patch passed
		_ Other Tests _
+1 💚	unit	4m 18s	tez-dag in the patch passed.
+1 💚	asflicense	0m 15s	The patch does not generate ASF License warnings.
		40m 57s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-138/1/artifact/out/Dockerfile
GITHUB PR	#138
JIRA Issue	TEZ-4321
Optional Tests	dupname asflicense javac javadoc unit spotbugs findbugs checkstyle compile
uname	Linux 0793fa824611 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/tez.sh
git revision	master / 464d86d
Default Java	Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
checkstyle	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-138/1/artifact/out/diff-checkstyle-tez-dag.txt
Test Results	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-138/1/testReport/
Max. process+thread count	209 (vs. ulimit of 5500)
modules	C: tez-dag U: tez-dag
Console output	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-138/1/console
versions	git=2.25.1 maven=3.6.3 findbugs=3.0.1
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[tez-yetus]

+1. Good catch.