[SPARK-49845][CORE] Make appArgs and environmentVariables optiona…

…l in REST API

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

This PR aims to make `appArgs` and `environmentVariables` fields optional in REST API.

### Why are the changes needed?

`appArgs` and `environmentVariables` became mandatory due to the Apache Mesos limitation at Spark 2.2.2. Technically, this is a revert of SPARK-22574.
- apache#19966

Since Apache Spark 4.0 removed Mesos support, we don't need these requirements.
- apache#43135

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

No because this is a relaxation of enforcement.

### How was this patch tested?

Pass the CIs with the revised test case.

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

No.

Closes apache#48316 from dongjoon-hyun/SPARK-49845.

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

core/src/main/scala/org/apache/spark/deploy/rest/StandaloneRestServer.scala

@@ -218,11 +218,12 @@ private[rest] class StandaloneSubmitRequestServlet(
     val (_, masterPort) = Utils.extractHostPortFromSparkUrl(masterUrl)
     val updatedMasters = masters.map(
       _.replace(s":$masterRestPort", s":$masterPort")).getOrElse(masterUrl)
-    val appArgs = request.appArgs
+    val appArgs = Option(request.appArgs).getOrElse(Array[String]())
     // Filter SPARK_LOCAL_(IP|HOSTNAME) environment variables from being set on the remote system.
     // In addition, the placeholders are replaced into the values of environment variables.
     val environmentVariables =
-      request.environmentVariables.filterNot(x => x._1.matches("SPARK_LOCAL_(IP|HOSTNAME)"))
+      Option(request.environmentVariables).getOrElse(Map.empty[String, String])
+        .filterNot(x => x._1.matches("SPARK_LOCAL_(IP|HOSTNAME)"))
         .map(x => (x._1, replacePlaceHolder(x._2)))
 
     // Construct driver description

core/src/main/scala/org/apache/spark/deploy/rest/SubmitRestProtocolRequest.scala

@@ -47,8 +47,6 @@ private[rest] class CreateSubmissionRequest extends SubmitRestProtocolRequest {
     super.doValidate()
     assert(sparkProperties != null, "No Spark properties set!")
     assertFieldIsSet(appResource, "appResource")
-    assertFieldIsSet(appArgs, "appArgs")
-    assertFieldIsSet(environmentVariables, "environmentVariables")
     assertPropertyIsSet("spark.app.name")
     assertPropertyIsBoolean(config.DRIVER_SUPERVISE.key)
     assertPropertyIsNumeric(config.DRIVER_CORES.key)

core/src/test/scala/org/apache/spark/deploy/rest/SubmitRestProtocolSuite.scala

@@ -87,8 +87,6 @@ class SubmitRestProtocolSuite extends SparkFunSuite {
     message.clientSparkVersion = "1.2.3"
     message.appResource = "honey-walnut-cherry.jar"
     message.mainClass = "org.apache.spark.examples.SparkPie"
-    message.appArgs = Array("two slices")
-    message.environmentVariables = Map("PATH" -> "/dev/null")
     val conf = new SparkConf(false)
     conf.set("spark.app.name", "SparkPie")
     message.sparkProperties = conf.getAll.toMap