[SPARK-35136] Remove initial null value of LiveStage.info

### What changes were proposed in this pull request?
To prevent potential NullPointerExceptions, this PR changes the `LiveStage` constructor to take `info` as a constructor parameter and adds a nullcheck in  `AppStatusListener.activeStages`.

### Why are the changes needed?
The `AppStatusListener.getOrCreateStage` would create a LiveStage object with the `info` field set to null and right after that set it to a specific StageInfo object. This can lead to a race condition when the `livestages` are read in between those calls. This could then lead to a null pointer exception in, for instance: `AppStatusListener.activeStages`.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Regular CI/CD tests

Closes #32233 from sander-goos/SPARK-35136-livestage.

Authored-by: Sander Goos <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

core/src/main/scala/org/apache/spark/status/AppStatusListener.scala

@@ -1018,7 +1018,7 @@ private[spark] class AppStatusListener(
    */
   def activeStages(): Seq[v1.StageData] = {
     liveStages.values.asScala
-      .filter(_.info.submissionTime.isDefined)
+      .filter(s => Option(s.info).exists(_.submissionTime.isDefined))
       .map(_.toApi())
       .toList
       .sortBy(_.stageId)
@@ -1179,7 +1179,7 @@ private[spark] class AppStatusListener(
 
   private def getOrCreateStage(info: StageInfo): LiveStage = {
     val stage = liveStages.computeIfAbsent((info.stageId, info.attemptNumber),
-      (_: (Int, Int)) => new LiveStage())
+      (_: (Int, Int)) => new LiveStage(info))
     stage.info = info
     stage
   }

core/src/main/scala/org/apache/spark/status/LiveEntity.scala

@@ -393,14 +393,13 @@ private class LiveExecutorStageSummary(
 
 }
 
-private class LiveStage extends LiveEntity {
+private class LiveStage(var info: StageInfo) extends LiveEntity {
 
   import LiveEntityHelpers._
 
   var jobs = Seq[LiveJob]()
   var jobIds = Set[Int]()
 
-  var info: StageInfo = null
   var status = v1.StageStatus.PENDING
 
   var description: Option[String] = None