[SPARK-32200][WEBUI] Redirect to the history page when accessed to /history on the HistoryServer without appliation id

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

This PR proposes to change the HistoryServer to redirect to the history page when we access to /history without application id.

### Why are the changes needed?

In the current master, status code 400 will be returned when we access to /history.
So I wonder it's better to redirect to the history page for the better UX.

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

Yes. In the current master, if we access to /history without application id, we will see like the following page.
![history-400](https://user-images.githubusercontent.com/4736016/86649650-e9105380-c01c-11ea-93bb-78fd8d2e6f7b.png)
After this change applied, we will be redirected to the history page.

### How was this patch tested?

New test added.

Closes #29016 from sarutak/history-redirect.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: sarutak

core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala

@@ -76,9 +76,7 @@ class HistoryServer(
       // attempt ID (separated by a slash).
       val parts = Option(req.getPathInfo()).getOrElse("").split("/")
       if (parts.length < 2) {
-        res.sendError(HttpServletResponse.SC_BAD_REQUEST,
-          s"Unexpected path info in request (URI = ${req.getRequestURI()}")
-        return
+        res.sendRedirect("/")
       }
 
       val appId = parts(1)

core/src/test/scala/org/apache/spark/deploy/history/HistoryServerSuite.scala

@@ -643,6 +643,19 @@ class HistoryServerSuite extends SparkFunSuite with BeforeAndAfter with Matchers
     val actualContentType = conn.getContentType
     assert(actualContentType === expectedContentType)
   }
+
+  test("Redirect to the root page when accessed to /history/") {
+    val port = server.boundPort
+    val url = new URL(s"http://localhost:$port/history/")
+    val conn = url.openConnection().asInstanceOf[HttpURLConnection]
+    conn.setRequestMethod("GET")
+    conn.setUseCaches(false)
+    conn.setDefaultUseCaches(false)
+    conn.setInstanceFollowRedirects(false)
+    conn.connect()
+    assert(conn.getResponseCode === 302)
+    assert(conn.getHeaderField("Location") === s"http://localhost:$port/")
+  }
 }
 
 object HistoryServerSuite {