[SPARK-28430][UI] Fix stage table rendering when some tasks' metrics are missing

## What changes were proposed in this pull request?

The Spark UI's stages table misrenders the input/output metrics columns when some tasks are missing input metrics. See the screenshot below for an example of the problem:

![image](https://user-images.githubusercontent.com/50748/61420042-a3abc100-a8b5-11e9-8a92-7986563ee712.png)

This is because those columns' are defined as

```scala
 {if (hasInput(stage)) {
  metricInfo(task) { m =>
    ...
   <td>....</td>
  }
}
```

where `metricInfo` renders the node returned by the closure in case metrics are defined or returns `Nil` in case metrics are not defined. If metrics are undefined then we'll fail to render the empty `<td></td>` tag, causing columns to become misaligned as shown in the screenshot.

To fix this, this patch changes this to

```scala
 {if (hasInput(stage)) {
  <td>{
    metricInfo(task) { m =>
      ...
     Unparsed(...)
    }
  }</td>
}
```

which is an idiom that's already in use for the shuffle read / write columns.

## How was this patch tested?

It isn't. I'm arguing for correctness because the modifications are consistent with rendering methods that work correctly for other columns.

Closes apache#25183 from JoshRosen/joshrosen/fix-task-table-with-partial-io-metrics.

Authored-by: Josh Rosen <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: JoshRosen

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -641,18 +641,22 @@ private[ui] class TaskPagedTable(
         <td>{accumulatorsInfo(task)}</td>
       }}
       {if (hasInput(stage)) {
-        metricInfo(task) { m =>
-          val bytesRead = Utils.bytesToString(m.inputMetrics.bytesRead)
-          val records = m.inputMetrics.recordsRead
-          <td>{bytesRead} / {records}</td>
-        }
+        <td>{
+          metricInfo(task) { m =>
+            val bytesRead = Utils.bytesToString(m.inputMetrics.bytesRead)
+            val records = m.inputMetrics.recordsRead
+            Unparsed(s"$bytesRead / $records")
+          }
+        }</td>
       }}
       {if (hasOutput(stage)) {
-        metricInfo(task) { m =>
-          val bytesWritten = Utils.bytesToString(m.outputMetrics.bytesWritten)
-          val records = m.outputMetrics.recordsWritten
-          <td>{bytesWritten} / {records}</td>
-        }
+        <td>{
+          metricInfo(task) { m =>
+            val bytesWritten = Utils.bytesToString(m.outputMetrics.bytesWritten)
+            val records = m.outputMetrics.recordsWritten
+            Unparsed(s"$bytesWritten / $records")
+          }
+        }</td>
       }}
       {if (hasShuffleRead(stage)) {
         <td class={TaskDetailsClassNames.SHUFFLE_READ_BLOCKED_TIME}>