[az fleet] get-credentials command fails with error with empty kubeconfig

[Ealianis]

Describe the bug

When calling az fleet get-credentials -g $groupName -n $fleetname. It is possible for an error to occur. When a user adds or removes clusters with tooling, such as kind.io, the cleanup of the KubeConfig can lead to removed root fields such as clusters. For example:

apiVersion: v1
# missing `clusters` key
kind: Config
preferences: {}

Regardless how the kubeconfig lands in such state, the command fails to handle it properly.

Related command

az fleet get-credentials

Errors

Traceback (most recent call last):
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/knack/cli.py", line 233, in invoke
cmd_result = self.invocation.execute(args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/init.py", line 664, in execute
raise ex
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/init.py", line 731, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/init.py", line 701, in _run_job
result = cmd_copy(params)
^^^^^^^^^^^^^^^^
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/init.py", line 334, in call
return self.handler(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/Cellar/azure-cli/2.58.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/command_operation.py", line 121, in handler
return op(**command_args)
^^^^^^^^^^^^^^^^^^
File "/Users/arvind/.azure/cliextensions/fleet/azext_fleet/custom.py", line 212, in get_credentials
print_or_merge_credentials(
File "/Users/arvind/.azure/cliextensions/fleet/azext_fleet/_helpers.py", line 47, in print_or_merge_credentials
_merge_kubernetes_configurations(
File "/Users/arvind/.azure/cliextensions/fleet/azext_fleet/_helpers.py", line 84, in _merge_kubernetes_configurations
_handle_merge(existing, addition, 'clusters', replace)
File "/Users/arvind/.azure/cliextensions/fleet/azext_fleet/_helpers.py", line 110, in _handle_merge
if existing[key] is None:
~~~~~~~~^^^^^
KeyError: 'clusters'

Issue script & Debug output

n/a the error message above is sufficient.

Expected behavior

The command should account for missing fields within the kubeconfig, it should not fail if the key does not exist.

Environment Summary

azure-cli 2.58.0

core 2.58.0
telemetry 1.1.0

Extensions:
fleet 1.0.5

Dependencies:
msal 1.26.0
azure-mgmt-resource 23.1.0b2

Additional context

No response

[yonzhan]

Thank you for opening this issue, we will look into it.

[Ealianis]

@yonzhan , I am a maintainer for the extension, please assign the bug to me.