Basic Ads 1.1.5

- Fix "Cannot redirect to an empty URL" crash on /ad/click/{nid} when an
  ad's link field resolves to an empty URL (e.g. route:<nolink>). The
  controller now logs a warning and returns 404 instead of 500, and
  skips click tracking for ads with a broken destination.
- Null-check loaded node in the click controller.
- README rewritten in drupal.org contrib style.