Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

lib/census_api.rb

@@ -1,6 +1,5 @@
 include DocumentParser
 class CensusApi
-
   def call(document_type, document_number)
     response = nil
     get_document_number_variants(document_type, document_number).each do |variant|
@@ -122,5 +121,4 @@ class CensusApi
     def stubbed_invalid_response
       { get_habita_datos_response: { get_habita_datos_return: { datos_habitante: {}, datos_vivienda: {}}}}
     end
-
 end