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/census_api.rb

@@ -22,6 +22,7 @@ class CensusApi
       str = data[:datos_habitante][:item][:fecha_nacimiento_string]
       day, month, year = str.match(/(\d\d?)\D(\d\d?)\D(\d\d\d?\d?)/)[1..3]
       return nil unless day.present? && month.present? && year.present?
+
       Time.zone.local(year.to_i, month.to_i, day.to_i).to_date
     end