2281081 - Port "Data value is empty (data_is_empty)" condition to 8.x

This is basically a copy of https://github.com/fago/rules/pull/66 with added unit test.
Related d.g.o. issue: https://www.drupal.org/node/2281081.
Drupal core now uses PHPunit 4, so we should be able to use the interface now. Please check.