fix cfg is none, load kube config error

[ACXLM]

This PR fixed
kubernetes/kubernetes#84503 changed the behaviour of kubectl config unset slightly that if no users left in kubeconfig after user run kubectl config unset users., the value of users field in kubeconfig would be set to null rather than [].

since users set to null, kubernetes.config.kube_config.load_kube_config() can't work, this pr set a default empty list to users

Which issue(s) this PR fixes:

Try to fixes
#183
#181

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please log a ticket with the Linux Foundation Helpdesk: https://support.linuxfoundation.org/
• Should you encounter any issues with the Linux Foundation Helpdesk, send a message to the backup e-mail support address at: login-issues@jira.linuxfoundation.org

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[k8s-ci-robot]

Welcome @ACXLM!

It looks like this is your first PR to kubernetes-client/python-base 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes-client/python-base has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[codecov-commenter]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #198 +/- ##
=======================================
Coverage 92.69% 92.69% =======================================
Files 13 13 Lines 1519 1519 =======================================
Hits 1408 1408 Misses 111 111 

Impacted Files	Coverage Δ
config/kube_config_test.py	95.59% <ø> (ø)
config/kube_config.py	83.48% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49ec060...2108e57. Read the comment docs.

[ACXLM]

/assign @roycaihw

[roycaihw]

Could you add a unit test for this behavior? Thanks

[ACXLM]

Could you add a unit test for this behavior? Thanks

since users set to None, load_config() run like the following error:

.....................config/kube_config_test.py:1345: DeprecationWarning: Please use assertEqual instead.
self.assertEquals(actual._config_persister.__name__, "save_changes")
....................................EEE...
======================================================================
ERROR: test_list_kube_config_contexts (__main__.TestKubeConfigMerger)
----------------------------------------------------------------------
Traceback (most recent call last):
File "config/kube_config_test.py", line 1545, in test_list_kube_config_contexts
contexts, active_context = list_kube_config_contexts(
File "/home/ac/Project/PythonWorksPaces/python-base/config/kube_config.py", line 714, in list_kube_config_contexts
loader = _get_kube_config_loader_for_yaml_file(config_file)
File "/home/ac/Project/PythonWorksPaces/python-base/config/kube_config.py", line 694, in _get_kube_config_loader_for_yaml_file
kcfg = KubeConfigMerger(filename)
File "/home/ac/Project/PythonWorksPaces/python-base/config/kube_config.py", line 650, in __init__
self.load_config(path)
File "/home/ac/Project/PythonWorksPaces/python-base/config/kube_config.py", line 667, in load_config
self._merge(item, config.get(item, []), path)
File "/home/ac/Project/PythonWorksPaces/python-base/config/kube_config.py", line 671, in _merge
for new_item in add_cfg:
TypeError: 'NoneType' object is not iterable

this PR can fix this error

[roycaihw]

/lgtm
/hold
LGTM once the pycodestyle failure is fixed: https://travis-ci.org/github/kubernetes-client/python-base/jobs/701111762

[k8s-ci-robot]

@ACXLM: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

Details

In response to this:

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[ACXLM]

/lgtm
/hold
LGTM once the pycodestyle failure is fixed: https://travis-ci.org/github/kubernetes-client/python-base/jobs/701111762

fixed pycodestyle test failure

[roycaihw]

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ACXLM, roycaihw

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [roycaihw]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment