Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

lib/evaluation_comment_email.rb

@@ -25,6 +25,7 @@ class EvaluationCommentEmail
 
     def related_users
       return [] if comment.commentable.nil?
+
       comment.commentable
              .admin_and_valuator_users_associated
              .reject { |associated_user| associated_user.user == comment.author }