Fix NPE in resolving k8s master url

```
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)
```