[SPARK-12466] Fix harmless NPE in tests

```
[info] ReplayListenerSuite:
[info] - Simple replay (58 milliseconds)
java.lang.NullPointerException
	at org.apache.spark.deploy.master.Master$$anonfun$asyncRebuildSparkUI$1.applyOrElse(Master.scala:982)
	at org.apache.spark.deploy.master.Master$$anonfun$asyncRebuildSparkUI$1.applyOrElse(Master.scala:980)
```
https://amplab.cs.berkeley.edu/jenkins/view/Spark-QA-Test/job/Spark-Master-SBT/4316/AMPLAB_JENKINS_BUILD_PROFILE=hadoop2.2,label=spark-test/consoleFull

This was introduced in #10284. It's harmless because the NPE is caused by a race that occurs mainly in `local-cluster` tests (but don't actually fail the tests).

Tested locally to verify that the NPE is gone.

Author: Andrew Or <andrew@databricks.com>

Closes #10417 from andrewor14/fix-harmless-npe.