[SPARK-16376][WEBUI][SPARK WEB UI][APP-ID] HTTP ERROR 500 when using rest api...

[SPARK-16376][WEBUI][SPARK WEB UI][APP-ID] HTTP ERROR 500 when using rest api "/applications//jobs" if array "stageIds" is empty

## What changes were proposed in this pull request?

Avoid error finding max of empty Seq when stageIds is empty. It does fix the immediate problem; I don't know if it results in meaningful output, but not an error at least.

## How was this patch tested?

Jenkins tests

Author: Sean Owen <sowen@cloudera.com>

Closes #14105 from srowen/SPARK-16376.

core/src/main/scala/org/apache/spark/status/api/v1/AllJobsResource.scala

@@ -68,7 +68,12 @@ private[v1] object AllJobsResource {
       listener: JobProgressListener,
       includeStageDetails: Boolean): JobData = {
     listener.synchronized {
-      val lastStageInfo = listener.stageIdToInfo.get(job.stageIds.max)
+      val lastStageInfo =
+        if (job.stageIds.isEmpty) {
+          None
+        } else {
+          listener.stageIdToInfo.get(job.stageIds.max)
+        }
       val lastStageData = lastStageInfo.flatMap { s =>
         listener.stageIdToData.get((s.stageId, s.attemptId))
       }