azure-cli: `az aks get-credentials` throws exception if ~/.kube/config does not contain the expected sections

Describe the bug az aks get-credentials expects ~/.kube/config to contain a clusters, users and contexts section. If even one of them is missing, then the command will bail out with a Python KeyError exception.

To Reproduce Create ~/.kube/config with the following contents:

apiVersion: v1
clusters: []
kind: Config

Then run:

az aks get-credentials --resource-group SOMETHING --name SOMETHING --overwrite-existing

Actual behavior

The command failed with an unexpected error. Here is the traceback:

'users'
Traceback (most recent call last):
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/knack/cli.py", line 206, in invoke
    cmd_result = self.invocation.execute(args)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/commands/__init__.py", line 603, in execute
    raise ex
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/commands/__init__.py", line 661, in _run_jobs_serially
    results.append(self._run_job(expanded_arg, cmd_copy))
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/commands/__init__.py", line 654, in _run_job
    six.reraise(*sys.exc_info())
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/six.py", line 693, in reraise
    raise value
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/commands/__init__.py", line 631, in _run_job
    result = cmd_copy(params)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/commands/__init__.py", line 306, in __call__
    return self.handler(*args, **kwargs)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/core/__init__.py", line 485, in default_command_handler
    return op(**command_args)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/command_modules/acs/custom.py", line 1887, in aks_get_credentials
    _print_or_merge_credentials(path, kubeconfig, overwrite_existing, context_name)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/command_modules/acs/custom.py", line 2775, in _print_or_merge_credentials
    merge_kubernetes_configurations(path, temp_path, overwrite_existing, context_name)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/command_modules/acs/custom.py", line 1178, in merge_kubernetes_configurations
    _handle_merge(existing, addition, 'users', replace)
  File "/usr/local/Cellar/azure-cli/2.0.74/libexec/lib/python3.7/site-packages/azure/cli/command_modules/acs/custom.py", line 1112, in _handle_merge
    if existing[key] is None:
KeyError: 'users'

To open an issue, please run: 'az feedback'

Expected behavior I expect the command to succeed, like this:

Merged "SOMETHING" as current context in ~/.kube/config

Environment summary I was able to reproduce this problem on two different environments.

Environment 1:

• Install method: Homebrew
• CLI version: 2.0.74
• OS: macOS Mojave
• Shell: bash

Environment 2:

• Install method: APT (via packages.microsoft.com)
• CLI version: 2.0.74
• OS: Ubuntu 16.04 (running inside a Docker container)

Additional context The problematic line is here: https://github.com/Azure/azure-cli/blob/0d4a040f56d481afa6b3d8cd1319b51bde9dd69a/src/azure-cli/azure/cli/command_modules/acs/custom.py#L1112

The code there attempts to check whether the config section exists, but does so in a wrong way. In Python, foo[key] raises an exception if the key doesn’t exist, so foo[key] is None makes no sense here. You want foo.get(key) is None which does not raise but returns None if the key does not exist.