fixup! Support exporting properties to task json

Fix a layering violation introduced when setting exported properties to
null when they are empty. This substitution is needed so that the json
field is not displayed on output which should only be of concern to the
TaskAttributeFactory and not to the Properties class. Move this null
setting to where the TaskAttribute is populated.

Change-Id: I39438932dc8ce44021e5aa91d1bec138608ade0a