heat-config script hook fails when an input value is None

[Marcin Lewandowski]

heat-config script hook fails when an input value is None (not given). This occurs when the script is a python script.
What's even worse about this problem, is that the resulting software deployment is marked as successful (should rather be error).

The relevant template excerpt:

resources:
  config:
    type: OS::Heat::SoftwareConfig
    properties:
      group: script
      inputs:
        - name: JSONTEST
          type: Json
      config: |
        #!/usr/bin/env python
        # ... the contents are irrelevant because never called

The relevant log excerpt (manually truncated for better readability):

...
os-collect-config: [2016-01-08 12:31:35,798] (heat-config) [INFO] JSONTEST=None
os-collect-config: [2016-01-08 12:31:35,798] (heat-config) [INFO] deploy_server_id=ea749d66-0122-4dd2-85b2-69178457a0dc
...
os-collect-config: [2016-01-08 12:31:35,799] (heat-config) [DEBUG] Running /var/lib/heat-config/heat-config-script/684245ec-52a7-45b7-a506-9d5b996c0147
os-collect-config: Traceback (most recent call last):
os-collect-config: File "/var/lib/heat-config/hooks/script", line 96, in <module>
os-collect-config: sys.exit(main(sys.argv))
os-collect-config: File "/var/lib/heat-config/hooks/script", line 66, in main
os-collect-config: stderr=subprocess.PIPE, env=env)
os-collect-config: File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
os-collect-config: errread, errwrite)
os-collect-config: File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
os-collect-config: raise child_exception
os-collect-config: TypeError: execve() arg 3 contains a non-string value
os-collect-config:
os-collect-config: [2016-01-08 12:31:35,807] (heat-config) [ERROR] Error running /var/lib/heat-config/hooks/script. [1]
...