Add example of checking registered variable for emptiness.

[forcewhisperer]

ISSUE TYPE

• Docs Pull Request

ANSIBLE VERSION

  ansible 2.0.1.0
  config file = 
  configured module search path = Default w/o overrides

SUMMARY

When testing for the emptiness of a registered variable, I came across a playbook using ‘when: registered_variable.stdout == “null”’. This was not working. After a little searching I discovered the check for emptiness worked like this ‘when: registered_variable.stdout == “”’. This change adds a short example at the end of the docs section on “conditionals” showing how to check a registered variable for emptiness.