[SPARK-26253][WEBUI] Task Summary Metrics Table on Stage Page shows empty table when no data is present

Task Summary Metrics Table on Stage Page shows empty table when no data is present instead of showing a message.

## What changes were proposed in this pull request?

Added a custom message to show on the task summary metrics table as well as executor summary table when no data is present.

## How was this patch tested?

**Before:**

![49335550-29277d00-f615-11e8-8e62-a953e76bcebf](https://user-images.githubusercontent.com/22228190/49361520-425a2780-f702-11e8-8df4-08862ab6ceb8.png)

**After:**

<img width="1413" alt="screen shot 2018-12-03 at 1 56 09 pm" src="https://user-images.githubusercontent.com/22228190/49362019-8699f780-f703-11e8-93e1-d02df6572923.png">

Closes #23205 from pgandhi999/SPARK-26253.

Authored-by: pgandhi <pgandhi@oath.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: pgandhi

core/src/main/resources/org/apache/spark/ui/static/stagepage.js

@@ -221,7 +221,10 @@ function createDataTableForTaskSummaryMetricsTable(taskSummaryMetricsTable) {
             "searching": false,
             "order": [[0, "asc"]],
             "bSort": false,
-            "bAutoWidth": false
+            "bAutoWidth": false,
+            "oLanguage": {
+                "sEmptyTable": "No tasks have reported metrics yet"
+            }
         };
         taskSummaryMetricsDataTable = $(taskMetricsTable).DataTable(taskConf);
     }
@@ -426,7 +429,10 @@ $(document).ready(function () {
                         }
                     ],
                     "order": [[0, "asc"]],
-                    "bAutoWidth": false
+                    "bAutoWidth": false,
+                    "oLanguage": {
+                        "sEmptyTable": "No data to show yet"
+                    }
                 }
                 var executorSummaryTableSelector =
                     $("#summary-executor-table").DataTable(executorSummaryConf);