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

[JoshRosen]

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:

This is because those columns' are defined as

 {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

 {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.

[SparkQA]

Test build #107803 has finished for PR 25183 at commit 7d8ae67.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Looks fine but usually if this patch involves UI changes, it attaches a screenshot after the fix though.

[gengliangwang]

LGTM

[dongjoon-hyun]

+1, LGTM. Thank you, @JoshRosen , @HyukjinKwon , @gengliangwang .
Merged to master/2.4/2.3.