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

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[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)

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[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

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

In response to this:

/retest

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[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

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[APPROVALNOTIFIER] This PR is APPROVED

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

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