[SPARK-3657] yarn alpha YarnRMClientImpl throws NPE appMasterRequest.setTrackingUrl starting spark-shell

@tgravescs reported this issue.

Following is quoted from @tgravescs' report.

YarnRMClientImpl.registerApplicationMaster can throw null pointer exception when setting the trackingurl if its empty:

    appMasterRequest.setTrackingUrl(new URI(uiAddress).getAuthority())


I hit this just start spark-shell without the tracking url set.

14/09/23 16:18:34 INFO yarn.YarnRMClientImpl: Connecting to ResourceManager at kryptonitered-jt1.red.ygrid.yahoo.com/98.139.154.99:8030
Exception in thread "main" java.lang.NullPointerException
        at org.apache.hadoop.yarn.proto.YarnServiceProtos$RegisterApplicationMasterRequestProto$Builder.setTrackingUrl(YarnServiceProtos.java:710)
        at org.apache.hadoop.yarn.api.protocolrecords.impl.pb.RegisterApplicationMasterRequestPBImpl.setTrackingUrl(RegisterApplicationMasterRequestPBImpl.java:132)
        at org.apache.spark.deploy.yarn.YarnRMClientImpl.registerApplicationMaster(YarnRMClientImpl.scala:102)
        at org.apache.spark.deploy.yarn.YarnRMClientImpl.register(YarnRMClientImpl.scala:55)
        at org.apache.spark.deploy.yarn.YarnRMClientImpl.register(YarnRMClientImpl.scala:38)
        at org.apache.spark.deploy.yarn.ApplicationMaster.registerAM(ApplicationMaster.scala:168)
        at org.apache.spark.deploy.yarn.ApplicationMaster.runExecutorLauncher(ApplicationMaster.scala:206)
        at org.apache.spark.deploy.yarn.ApplicationMaster.run(ApplicationMaster.scala:120)