HDFS-16693. RBF: fix NPE in Quota

[WeisonWei]

Description of PR

How was this patch tested?

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?

[ZanderXu]

Thanks @WeisonWei for your contribution. Can you add a UT to reproduce it?

If DFS_ROUTER_QUOTA_ENABLE=false, doesn't getEachQuotaUsage return IOException?

Map<RemoteLocation, QuotaUsage> getEachQuotaUsage(String path)
      throws IOException {
    rpcServer.checkOperation(OperationCategory.READ);
    if (!router.isQuotaEnabled()) {
      throw new IOException("The quota system is disabled in Router.");
    }
   ...
}

[hadoop-yetus]

:broken_heart: -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	0m 38s		Docker mode activated.
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+0 :ok:	codespell	0m 0s		codespell was not available.
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+1 :green_heart:	@author	0m 0s		The patch does not contain any @author tags.
-1 :x:	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 :green_heart:	mvninstall	38m 29s		trunk passed
+1 :green_heart:	compile	1m 1s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 :green_heart:	compile	0m 58s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
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+1 :green_heart:	javadoc	1m 17s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 :green_heart:	spotbugs	1m 47s		trunk passed
+1 :green_heart:	shadedclient	23m 43s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 :green_heart:	mvninstall	0m 42s		the patch passed
+1 :green_heart:	compile	0m 49s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 :green_heart:	javac	0m 49s		the patch passed
+1 :green_heart:	compile	0m 37s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 :green_heart:	javac	0m 37s		the patch passed
+1 :green_heart:	blanks	0m 0s		The patch has no blanks issues.
+1 :green_heart:	checkstyle	0m 23s		the patch passed
+1 :green_heart:	mvnsite	0m 39s		the patch passed
+1 :green_heart:	javadoc	0m 41s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 :green_heart:	javadoc	0m 58s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 :green_heart:	spotbugs	1m 26s		the patch passed
+1 :green_heart:	shadedclient	20m 37s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 :green_heart:	unit	23m 0s		hadoop-hdfs-rbf in the patch passed.
+1 :green_heart:	asflicense	0m 51s		The patch does not generate ASF License warnings.
		123m 23s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4633/1/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/4633
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 61805d2ecd50 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
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Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
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Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4633/1/testReport/
Max. process+thread count	2637 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs-rbf U: hadoop-hdfs-project/hadoop-hdfs-rbf
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4633/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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