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.

app/controllers/management/document_verifications_controller.rb

@@ -45,6 +45,7 @@ class Management::DocumentVerificationsController < Management::BaseController
 
     def clean_document_number
       return if params[:document_verification][:document_number].blank?
+
       params[:document_verification][:document_number] = params[:document_verification][:document_number].gsub(/[^a-z0-9]+/i, "").upcase
     end
 end