[SPARK-23330][WEBUI] Spark UI SQL executions page throws NPE

## What changes were proposed in this pull request?

Spark SQL executions page throws the following error and the page crashes:
```
HTTP ERROR 500
Problem accessing /SQL/. Reason:

Server Error
Caused by:
java.lang.NullPointerException
at scala.collection.immutable.StringOps$.length$extension(StringOps.scala:47)
at scala.collection.immutable.StringOps.length(StringOps.scala:47)
at scala.collection.IndexedSeqOptimized$class.isEmpty(IndexedSeqOptimized.scala:27)
at scala.collection.immutable.StringOps.isEmpty(StringOps.scala:29)
at scala.collection.TraversableOnce$class.nonEmpty(TraversableOnce.scala:111)
at scala.collection.immutable.StringOps.nonEmpty(StringOps.scala:29)
at org.apache.spark.sql.execution.ui.ExecutionTable.descriptionCell(AllExecutionsPage.scala:182)
at org.apache.spark.sql.execution.ui.ExecutionTable.row(AllExecutionsPage.scala:155)
at org.apache.spark.sql.execution.ui.ExecutionTable$$anonfun$8.apply(AllExecutionsPage.scala:204)
at org.apache.spark.sql.execution.ui.ExecutionTable$$anonfun$8.apply(AllExecutionsPage.scala:204)
at org.apache.spark.ui.UIUtils$$anonfun$listingTable$2.apply(UIUtils.scala:339)
at org.apache.spark.ui.UIUtils$$anonfun$listingTable$2.apply(UIUtils.scala:339)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
at scala.collection.AbstractTraversable.map(Traversable.scala:104)
at org.apache.spark.ui.UIUtils$.listingTable(UIUtils.scala:339)
at org.apache.spark.sql.execution.ui.ExecutionTable.toNodeSeq(AllExecutionsPage.scala:203)
at org.apache.spark.sql.execution.ui.AllExecutionsPage.render(AllExecutionsPage.scala:67)
at org.apache.spark.ui.WebUI$$anonfun$2.apply(WebUI.scala:82)
at org.apache.spark.ui.WebUI$$anonfun$2.apply(WebUI.scala:82)
at org.apache.spark.ui.JettyUtils$$anon$3.doGet(JettyUtils.scala:90)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:848)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:584)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1180)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:512)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1112)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:213)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:134)
at org.eclipse.jetty.server.Server.handle(Server.java:534)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:320)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:251)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:283)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:108)
at org.eclipse.jetty.io.SelectChannelEndPoint$2.run(SelectChannelEndPoint.java:93)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.executeProduceConsume(ExecuteProduceConsume.java:303)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.produceConsume(ExecuteProduceConsume.java:148)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.run(ExecuteProduceConsume.java:136)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:671)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:589)
at java.lang.Thread.run(Thread.java:748)
```

One of the possible reason that this page fails may be the `SparkListenerSQLExecutionStart` event get dropped before processed, so the execution description and details don't get updated.
This was not a issue in 2.2 because it would ignore any job start event that arrives before the corresponding execution start event, which doesn't sound like a good decision.

We shall try to handle the null values in the front page side, that is, try to give a default value when `execution.details` or `execution.description` is null.
Another possible approach is not to spill the `LiveExecutionData` in `SQLAppStatusListener.update(exec: LiveExecutionData)` if `exec.details` is null. This is not ideal because this way you will not see the execution if `SparkListenerSQLExecutionStart` event is lost, because `AllExecutionsPage` only read executions from KVStore.

## How was this patch tested?

After the change, the page shows the following:
![image](https://user-images.githubusercontent.com/4784782/35775480-28cc5fde-093e-11e8-8ccc-f58c2ef4a514.png)

Author: Xingbo Jiang <[email protected]>

Closes #20502 from jiangxb1987/executionPage.

sql/core/src/main/scala/org/apache/spark/sql/execution/ui/AllExecutionsPage.scala

@@ -179,7 +179,7 @@ private[ui] abstract class ExecutionTable(
   }
 
   private def descriptionCell(execution: SQLExecutionUIData): Seq[Node] = {
-    val details = if (execution.details.nonEmpty) {
+    val details = if (execution.details != null && execution.details.nonEmpty) {
       <span onclick="clickDetail(this)" class="expand-details">
         +details
       </span> ++
@@ -190,8 +190,10 @@ private[ui] abstract class ExecutionTable(
       Nil
     }
 
-    val desc = {
+    val desc = if (execution.description != null && execution.description.nonEmpty) {
       <a href={executionURL(execution.executionId)}>{execution.description}</a>
+    } else {
+      <a href={executionURL(execution.executionId)}>{execution.executionId}</a>
     }
 
     <div>{desc} {details}</div>