[k8s] Fix show-gpus when running with incluster auth

[romilbhardwaj]

sky show-gpus when running with in-cluster auth would previously return empty list since context=None was not handled correctly. This PR fixes it to use in-cluster auth when context=None in sky show-gpus.

Tested:

• Base case - when kubeconfig does not exist on local laptop (context will be None, not incluster = should return no GPUs)
• server from restapi branch running in a k8s pod; both sky show-gpus and sky show-gpus L4work (context will be None but now we handle in-cluster case correctly).

[Michaelvll]

LGTM! Thanks @romilbhardwaj !