What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

Some content from commit 261af9b in PR #20 was lost after a rebase.

What is the current behavior?
Some optional keys are saved with a null value in flogo.json file:

  "properties": null,
  "channels": null,
  "actions": null

What is the new behavior?
If the key is optional and not present, it is not saved in flogo.json