Fix deserialization of nil relationships

failing test

use try for when the assoc_data is possibly nil

rubocop test/action_controller/json_api/deserialization_test.rb -a

attempt to work on rails-master

account for rails/master having  instead of nil for assoc_data

added changelog

lib/active_model_serializers/adapter/json_api/deserialization.rb

@@ -188,7 +188,9 @@ module ActiveModelSerializers
             end
 
           polymorphic = (options[:polymorphic] || []).include?(assoc_name.to_sym)
-          hash["#{prefix_key}_type".to_sym] = assoc_data[:type] if polymorphic
+          if polymorphic
+            hash["#{prefix_key}_type".to_sym] = assoc_data.present? ? assoc_data[:type] : nil
+          end
 
           hash
         end