Return invalid response when document_type or document_number are empty

Probably this case is not real for production environments where those
arguments will always be fullfilled but seems to be interesting for
testing environment where this method is being called when those
paremeters where empty.

lib/census_caller.rb

@@ -1,5 +1,7 @@
 class CensusCaller
   def call(document_type, document_number, date_of_birth, postal_code)
+    return Response.new if document_number.blank? || document_type.blank?
+
     if Setting["feature.remote_census"].present?
       response = RemoteCensusApi.new.call(document_type, document_number, date_of_birth, postal_code)
     else
@@ -9,4 +11,10 @@ class CensusCaller
 
     response
   end
+
+  class Response
+    def valid?
+      false
+    end
+  end
 end