ci: compliance: add a PR description check step

Add an extra workflow step to block PRs with an empty description so that
these gets flagged automatically and reviewers don't have to ask for it.

Add a message in the actual fail step as well so it's easier for the
user to figure out what went wrong, even in the existing compliance
check case.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>