Empty parameters list passes 'make lint', fails on import

[LindsayHill]

If I have an action.yaml file like this:

name: action
runner_type: run-python
description: List Some Stuff
enabled: true
entry_point: action_script.py
parameters:

Then it will pass make lint, but it will fail when registering with an error similar to this:

Traceback (most recent call last):
  File "/usr/bin/st2-register-content", line 22, in <module>
    sys.exit(content_loader.main(sys.argv[1:]))
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/content/bootstrap.py", line 354, in main
    register_content()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/content/bootstrap.py", line 326, in register_content
    register_actions()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/content/bootstrap.py", line 195, in register_actions
    raise e
AttributeError: 'NoneType' object has no attribute 'iteritems'

It would be better if this error was detected when running make lint, rather than at registration time.