ISSUE-8673: Tests of pulsar-functions-runtime are not passing on 2.7.0rc1 - if you do not have a local Kubernets environment

[sijie]

Original Issue: apache#8673

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Describe the bug
Tests are not passing due to an error on Kubernetes client runtime, it created to a default configuration, failing with a NullPointerException.

To Reproduce

• build pulsar from 2.7.0rc1 sources
• run tests in pulsar-functions-runtime

Expected behavior
Tests are passing

Desktop (please complete the following information):

• OS: Fedora
• Maven 3.6.3
• JDK 1.8 (OpenJDK)

Additional context

[ERROR] testResources(org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeTest)  Time elapsed: 0.011 s  <<< FAILURE!
java.lang.RuntimeException: java.lang.NullPointerException
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeFactory.initialize(KubernetesRuntimeFactory.java:211)
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeTest.createKubernetesRuntimeFactory(KubernetesRuntimeTest.java:217)
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeTest.createKubernetesRuntimeFactory(KubernetesRuntimeTest.java:225)
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeTest.testResources(KubernetesRuntimeTest.java:340)
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeTest.testResources(KubernetesRuntimeTest.java:324)
	at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
	at org.testng.internal.InvokeMethodRunnable.runOne(InvokeMethodRunnable.java:45)
	at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:73)
	at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:11)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
	at io.kubernetes.client.util.ClientBuilder.kubeconfig(ClientBuilder.java:259)
	at io.kubernetes.client.util.ClientBuilder.getClientBuilder(ClientBuilder.java:108)
	at io.kubernetes.client.util.ClientBuilder.standard(ClientBuilder.java:87)
	at io.kubernetes.client.util.ClientBuilder.standard(ClientBuilder.java:79)
	at io.kubernetes.client.util.Config.defaultClient(Config.java:113)
	at org.apache.pulsar.functions.runtime.kubernetes.KubernetesRuntimeFactory.setupClient(KubernetesRuntimeFactory.java:339)
	at org.apache.pul