Scenario tests all failing trying to decode `None` as JSON

I’m trying to add to the scenario tests for ACS/AKS, but they all error out for me now. This same setup was working fine previously, so I’m wondering if some underlying library broke, or if any az folks have ideas.

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Environment summary

macOS High Sierra 10.13.1, python 3.6.3 installed with brew, azure-cli checked out from GitHub and virtualenv configured as per developer docs. I’m running the HEAD of the dev branch. Here’s one example, but all ScenarioTests fail similarly:

$ run_tests --module acs --test test_aks_commands.AzureKubernetesServiceScenarioTest.test_aks_create_default_service
Run automation
Modules: acs
Drive test by unittest
E
======================================================================
ERROR: test_aks_create_default_service (azure.cli.command_modules.acs.tests.test_aks_commands.AzureKubernetesServiceScenarioTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/matt/Projects/azure-cli/env/lib/python3.6/site-packages/azure_devtools/scenario_tests/preparers.py", line 60, in _preparer_wrapper
    fn(test_class_instance, **kwargs)
  File "/Users/matt/Projects/azure-cli/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/tests/test_aks_commands.py", line 30, in test_aks_create_default_service
    JMESPathCheck('properties.provisioningState', 'Succeeded')
  File "/Users/matt/Projects/azure-cli/src/azure-cli-testsdk/azure/cli/testsdk/base.py", line 76, in cmd
    return execute(command, expect_failure=expect_failure).assert_with_checks(checks)
  File "/Users/matt/Projects/azure-cli/src/azure-cli-testsdk/azure/cli/testsdk/base.py", line 128, in assert_with_checks
    c(self)
  File "/Users/matt/Projects/azure-cli/src/azure-cli-testsdk/azure/cli/testsdk/checkers.py", line 34, in __call__
    json_value = execution_result.get_output_in_json()
  File "/Users/matt/Projects/azure-cli/src/azure-cli-testsdk/azure/cli/testsdk/base.py", line 134, in get_output_in_json
    self.json_value = json.loads(self.output)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

----------------------------------------------------------------------
Ran 1 test in 12.038s

FAILED (errors=1)

[derekbekoe]

I would rather not have this in the tests.

[mboersma]

I could add an explicit -o json to the scenario tests as a workaround.