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/remote_census_api.rb

@@ -17,6 +17,7 @@ class RemoteCensusApi
     def extract_value(path_value)
       path = parse_response_path(path_value)
       return nil unless path.present?
+
       @body.dig(*path)
     end
 
@@ -29,8 +30,10 @@ class RemoteCensusApi
       path_value = Setting["remote_census.response.date_of_birth"]
       str = extract_value(path_value)
       return nil unless str.present?
+
       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
 
@@ -111,6 +114,7 @@ class RemoteCensusApi
       to_set = path.empty? ? structure : structure.dig(*path)
 
       return unless to_set
+
       to_set[final_key] = value
     end