- Initial attempt at implementing the data value is empty condition.

Hey Fago,

It was great working with you on Rule at the DrupalCon Austin sprint. I want to get more involved with the Rules team, but I need to clear some client projects off my plate first. In the mean time, I didn't want to hold up https://drupal.org/node/2281081 and also didn't want my work to go to waste, so it is contained within this pull request. Let me know if you have any questions.

Cheers,

jantoine