Throw a meanful exception error instead of NPE when no plurals are set

[lordofthejars]

Is your enhancement related to a problem? Please describe

When you create manually a ResourceDefinitionContext and you don't set the plurals field a NPE exception is thrown when you get it using the Kubernetes client

final ResourceDefinitionContext resourceDefinitionContext = new ResourceDefinitionContext.Builder()
            .withGroup("networking.istio.io")
            .withVersion("v1alpha3")
            .withKind("VirtualService")
            .build();

        System.out.println("Namespace: " + namespace);

        final GenericKubernetesResource virtualResource = kubernetesClient.genericKubernetesResources(resourceDefinitionContext).inNamespace(namespace).withName(virtualServiceName).get();

Caused by: java.lang.NullPointerException
	at java.base/java.net.URI$Parser.parse(URI.java:3129)
	at java.base/java.net.URI.<init>(URI.java:623)
	at io.fabric8.kubernetes.client.utils.URLUtils.join(URLUtils.java:50)
	at io.fabric8.kubernetes.client.dsl.base.OperationSupport.getNamespacedUrl(OperationSupport.java:164)
	at io.fabric8.kubernetes.client.dsl.base.OperationSupport.getNamespacedUrl(OperationSupport.java:169)
	at io.fabric8.kubernetes.client.dsl.base.BaseOperation.getCompleteResourceUrl(BaseOperation.java:818)
	at io.fabric8.kubernetes.client.dsl.base.BaseOperation.getMandatory(BaseOperation.java:188)
	at io.fabric8.kubernetes.client.dsl.base.BaseOperation.get(BaseOperation.java:156)
	at io.fabric8.kubernetes.client.dsl.base.BaseOperation.get(BaseOperation.java:89)

Describe the solution you'd like

Inform why it is failing.

Describe alternatives you've considered

No response

Additional context

No response

[shawkins]

Let's make sure that kind is required, then default to default to pluralizing the kind if plural is not specified.