Handle explicit nil argument in #update_approvals [#73573]

This works around a situation I ran into when testing on the NU fork: If the none of the products on the access list form have scheduling groups, `param[:product_access_group]` is `nil`. This explicit `nil` would get passed as the `access_group_hash` argument to `#update_approvals`, and would throw an exception when trying to use it as a hash.