[SPARK-38579][SQL][WEBUI] Requesting Restful API can cause NullPointe…

…rException

### What changes were proposed in this pull request?

Added null check for `exec.metricValues`.

### Why are the changes needed?

When requesting Restful API  {baseURL}/api/v1/applications/$appId/sql/$executionId which is introduced by this PR apache#28208, it can cause NullPointerException. The root cause is, when calling method doUpdate() of `LiveExecutionData`, `metricsValues` can be null. Then, when statement `printableMetrics(graph.allNodes, exec.metricValues)` is executed, it will throw NullPointerException.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Tested manually.

Closes apache#35884 from yym1995/fix-npe.

Lead-authored-by: Yimin <yimin.y@outlook.com>
Co-authored-by: Yimin Yang <26797163+yym1995@users.noreply.github.com>
Signed-off-by: Yuming Wang <yumwang@ebay.com>
(cherry picked from commit 99992a4)
Signed-off-by: Yuming Wang <yumwang@ebay.com>

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

@@ -94,7 +94,11 @@ private[v1] class SqlResource extends BaseAppResource {
 
     val duration = exec.completionTime.getOrElse(new Date()).getTime - exec.submissionTime
     val planDetails = if (planDescription) exec.physicalPlanDescription else ""
-    val nodes = if (details) printableMetrics(graph.allNodes, exec.metricValues) else Seq.empty
+    val nodes = if (details) {
+      printableMetrics(graph.allNodes, Option(exec.metricValues).getOrElse(Map.empty))
+    } else {
+      Seq.empty
+    }
     val edges = if (details) graph.edges else Seq.empty
 
     new ExecutionData(