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/active_model/dates.rb

@@ -5,6 +5,7 @@ module ActiveModel::Dates
                        attrs["#{field}(3i)"]
 
     return nil unless day.present? && month.present? && year.present?
+
     Date.new(year.to_i, month.to_i, day.to_i)
   end