[SPARK-30531][WEB UI] Do not render plan viz when it exists already

### What changes were proposed in this pull request?
When you save a Spark UI SQL query page to disk and then display the html file with your browser, the query plan will be rendered a second time. This change avoids rendering the plan visualization when it exists already.

This is master:
![grafik](https://user-images.githubusercontent.com/44700269/72543429-fcb8d980-3885-11ea-82aa-c0b3638847e5.png)

And with the fix:
![grafik](https://user-images.githubusercontent.com/44700269/72543641-57523580-3886-11ea-8cdf-5fb0cdffa983.png)

### Why are the changes needed?
The duplicate query plan is unexpected and redundant.

### Does this PR introduce any user-facing change?
No.

### How was this patch tested?
Manually tested. Testing this in a reproducible way requires a running browser or HTML rendering engine that executes the JavaScript.

Closes #27238 from EnricoMi/branch-sql-ui-duplicate-plan.

Authored-by: Enrico Minack <github@enrico.minack.dev>
Signed-off-by: Sean Owen <srowen@gmail.com>

sql/core/src/main/resources/org/apache/spark/sql/execution/ui/static/spark-sql-viz.js

@@ -20,6 +20,10 @@ var PlanVizConstants = {
   svgMarginY: 16
 };
 
+function shouldRenderPlanViz() {
+  return planVizContainer().selectAll("svg").empty();
+}
+
 function renderPlanViz() {
   var svg = planVizContainer().append("svg");
   var metadata = d3.select("#plan-viz-metadata");

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

@@ -116,7 +116,7 @@ class ExecutionPage(parent: SQLTab) extends WebUIPage("execution") with Logging
         {metadata}
       </div>
       {planVisualizationResources(request)}
-      <script>$(function() {{ renderPlanViz(); }})</script>
+      <script>$(function() {{ if (shouldRenderPlanViz()) {{ renderPlanViz(); }} }})</script>
     </div>
   }