Add defaults for admin_metadata and customer_metadata

If the default value is not passed the attributes will be nil instead of an empty json object
so the empty json would now be default for the attributes.

Author: JustShah

core/app/models/concerns/spree/metadata.rb

@@ -5,8 +5,8 @@ module Metadata
     extend ActiveSupport::Concern
 
     included do
-      attribute :customer_metadata, :json
-      attribute :admin_metadata, :json
+      attribute :customer_metadata, :json, default: {}
+      attribute :admin_metadata, :json, default: {}
 
       validate :validate_metadata_limits
     end