Validate resource_id

[takestar15]

If user clicks 'Add Refinery Page' without checking any pages, it returns error.

https://github.com/pylonweb/refinerycms-menus/blob/master/app/controllers/refinery/menus/admin/menu_links_controller.rb#L15
I think we have to validate params[:menu_link][:resource_id].

Request Sample:
commit: Add Refinery Page
menu_link[refinery_menu_id]: 10
menu_link[resource_type]: refinery_page
Error:
create action returns one menu_link, but it's not valid because label field is empty.