Ensure options is always a hash

#17802 added a line that called []
on options and if nil, it would raise NoMethodError where it should have
returned nil

app/models/custom_button.rb

@@ -1,7 +1,7 @@
 class CustomButton < ApplicationRecord
   has_one :resource_action, :as => :resource, :dependent => :destroy, :autosave => true
 
-  serialize :options
+  serialize :options, Hash
   serialize :visibility_expression
   serialize :enablement_expression
   serialize :visibility