[SPARK-50021][CORE][UI][3.5] Fix ApplicationPage to hide App UI lin…

…ks when UI is disabled

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

This PR aims to fix `ApplicationPage` to hide UI link when UI is disabled.

### Why are the changes needed?

Previously, Spark throws `HTTP ERROR 500 java.lang.IllegalArgumentException: Invalid URI host: null (authority: null)` like the following

**1. PREPARATION**
```
$ cat conf/spark-defaults.conf
spark.ui.reverseProxy true
spark.ui.reverseProxyUrl http://localhost:8080

$ sbin/start-master.sh

$ sbin/start-worker.sh spark://$(hostname):7077

$ bin/spark-shell --master spark://$(hostname):7077 -c spark.ui.enabled=false
```

**2. BEFORE**
<img width="496" alt="Screenshot 2024-10-17 at 21 24 32" src="https://github.com/user-attachments/assets/9884790c-a294-4e61-b630-7758c5532afc">

<img width="1002" alt="Screenshot 2024-10-17 at 21 24 51" src="https://github.com/user-attachments/assets/f1e3a121-37ba-4525-a433-21ad15402edf">

**3. AFTER**
<img width="493" alt="Screenshot 2024-10-17 at 21 22 26" src="https://github.com/user-attachments/assets/7a1ef578-3d9f-495e-9545-6edd26b4d565">

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

Yes, but previously it was a broken link.

### How was this patch tested?

Pass the CIs with the newly added test case.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#48547 from dongjoon-hyun/SPARK-50021-3.5.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

core/src/main/scala/org/apache/spark/deploy/master/ui/ApplicationPage.scala

@@ -21,6 +21,8 @@ import javax.servlet.http.HttpServletRequest
 
 import scala.xml.Node
 
+import org.apache.commons.lang3.StringUtils
+
 import org.apache.spark.deploy.DeployMessages.{MasterStateResponse, RequestMasterState}
 import org.apache.spark.deploy.ExecutorState
 import org.apache.spark.deploy.StandaloneResourceUtils.{formatResourceRequirements, formatResourcesAddresses}
@@ -93,10 +95,14 @@ private[ui] class ApplicationPage(parent: MasterWebUI) extends WebUIPage("app")
             <li><strong>State:</strong> {app.state}</li>
             {
               if (!app.isFinished) {
-                <li><strong>
-                    <a href={UIUtils.makeHref(parent.master.reverseProxy,
-                      app.id, app.desc.appUiUrl)}>Application Detail UI</a>
-                </strong></li>
+                if (StringUtils.isBlank(app.desc.appUiUrl)) {
+                  <li><strong>Application UI:</strong> Disabled</li>
+                } else {
+                  <li><strong>
+                      <a href={UIUtils.makeHref(parent.master.reverseProxy,
+                        app.id, app.desc.appUiUrl)}>Application Detail UI</a>
+                  </strong></li>
+                }
               }
             }
           </ul>

core/src/test/scala/org/apache/spark/deploy/master/ui/ApplicationPageSuite.scala

@@ -0,0 +1,73 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.deploy.master.ui
+
+import java.util.Date
+import javax.servlet.http.HttpServletRequest
+
+import org.mockito.Mockito.{mock, when}
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.deploy.ApplicationDescription
+import org.apache.spark.deploy.DeployMessages.{MasterStateResponse, RequestMasterState}
+import org.apache.spark.deploy.master.{ApplicationInfo, ApplicationState, Master}
+import org.apache.spark.resource.ResourceProfile
+import org.apache.spark.rpc.RpcEndpointRef
+
+class ApplicationPageSuite extends SparkFunSuite {
+
+  private val master = mock(classOf[Master])
+
+  private val rp = new ResourceProfile(Map.empty, Map.empty)
+  private val desc = ApplicationDescription("name", Some(4), null, "appUiUrl", rp)
+  private val descWithoutUI = ApplicationDescription("name", Some(4), null, "", rp)
+  private val appFinished = new ApplicationInfo(0, "app-finished", desc, new Date, null, 1)
+  appFinished.markFinished(ApplicationState.FINISHED)
+  private val appLive = new ApplicationInfo(0, "app-live", desc, new Date, null, 1)
+  private val appLiveWithoutUI =
+    new ApplicationInfo(0, "app-live-without-ui", descWithoutUI, new Date, null, 1)
+
+  private val state = mock(classOf[MasterStateResponse])
+  when(state.completedApps).thenReturn(Array(appFinished))
+  when(state.activeApps).thenReturn(Array(appLive, appLiveWithoutUI))
+
+  private val rpc = mock(classOf[RpcEndpointRef])
+  when(rpc.askSync[MasterStateResponse](RequestMasterState)).thenReturn(state)
+
+  private val masterWebUI = mock(classOf[MasterWebUI])
+  when(masterWebUI.master).thenReturn(master)
+  when(masterWebUI.masterEndpointRef).thenReturn(rpc)
+
+  test("SPARK-45774: Application Detail UI") {
+    val request = mock(classOf[HttpServletRequest])
+    when(request.getParameter("appId")).thenReturn("app-live")
+
+    val result = new ApplicationPage(masterWebUI).render(request).toString()
+    assert(result.contains("Application Detail UI"))
+    assert(!result.contains("Application History UI"))
+  }
+
+  test("SPARK-50021: Application Detail UI is empty when spark.ui.enabled=false") {
+    val request = mock(classOf[HttpServletRequest])
+    when(request.getParameter("appId")).thenReturn("app-live-without-ui")
+
+    val result = new ApplicationPage(masterWebUI).render(request).toString()
+    assert(result.contains("Application UI:</strong> Disabled"))
+    assert(!result.contains("Application History UI"))
+  }
+}