Always return an enumerator

lib/active_model/serializer.rb

@@ -332,10 +332,9 @@ module ActiveModel
     # @param [JSONAPI::IncludeDirective] include_directive (defaults to the
     #   +default_include_directive+ config value when not provided)
     # @return [Enumerator<Association>]
-    #
     def associations(include_directive = ActiveModelSerializers.default_include_directive, include_slice = nil)
       include_slice ||= include_directive
-      return unless object
+      return Enumerator.new unless object
 
       Enumerator.new do |y|
         self.class._reflections.values.each do |reflection|