[SPARK-30970][K8S][CORE][2.4] Fix NPE while resolving k8s master url

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

backport #27721
```
bin/spark-sql --master  k8s:///https://kubernetes.docker.internal:6443 --conf spark.kubernetes.container.image=yaooqinn/spark:v2.4.4
Exception in thread "main" java.lang.NullPointerException
	at org.apache.spark.util.Utils$.checkAndGetK8sMasterUrl(Utils.scala:2739)
	at org.apache.spark.deploy.SparkSubmit.prepareSubmitEnvironment(SparkSubmit.scala:261)
	at org.apache.spark.deploy.SparkSubmit.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:774)
	at org.apache.spark.deploy.SparkSubmit.doRunMain$1(SparkSubmit.scala:161)
	at org.apache.spark.deploy.SparkSubmit.submit(SparkSubmit.scala:184)
	at org.apache.spark.deploy.SparkSubmit.doSubmit(SparkSubmit.scala:86)
	at org.apache.spark.deploy.SparkSubmit$$anon$2.doSubmit(SparkSubmit.scala:920)
	at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:929)
	at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)
```
Althrough `k8s:///https://kubernetes.docker.internal:6443` is a wrong master url but should not throw npe
The `case null` will never be touched.
https://github.com/apache/spark/blob/3f4060c340d6bac412e8819c4388ccba226efcf3/core/src/main/scala/org/apache/spark/util/Utils.scala#L2772-L2776

### Why are the changes needed?

bug fix

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

no

### How was this patch tested?

add ut case

Closes #27736 from yaooqinn/SPARK-30970-2.

Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -2736,19 +2736,16 @@ private[spark] object Utils extends Logging {
     }
 
     val masterScheme = new URI(masterWithoutK8sPrefix).getScheme
-    val resolvedURL = masterScheme.toLowerCase match {
-      case "https" =>
+
+    val resolvedURL = Option(masterScheme).map(_.toLowerCase(Locale.ROOT)) match {
+      case Some("https") =>
         masterWithoutK8sPrefix
-      case "http" =>
+      case Some("http") =>
         logWarning("Kubernetes master URL uses HTTP instead of HTTPS.")
         masterWithoutK8sPrefix
-      case null =>
-        val resolvedURL = s"https://$masterWithoutK8sPrefix"
-        logInfo("No scheme specified for kubernetes master URL, so defaulting to https. Resolved " +
-          s"URL is $resolvedURL.")
-        resolvedURL
       case _ =>
-        throw new IllegalArgumentException("Invalid Kubernetes master scheme: " + masterScheme)
+        throw new IllegalArgumentException("Invalid Kubernetes master scheme: " + masterScheme
+          + " found in URL: " + masterWithoutK8sPrefix)
     }
 
     s"k8s://$resolvedURL"

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -1167,6 +1167,10 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
     intercept[IllegalArgumentException] {
       Utils.checkAndGetK8sMasterUrl("k8s://foo://host:port")
     }
+
+    intercept[IllegalArgumentException] {
+      Utils.checkAndGetK8sMasterUrl("k8s:///https://host:port")
+    }
   }
 
   object MalformedClassObject {