Spec validate_uniqueness_of test scoped with conditional

I have a model where I'm validating a field conditionally. It looks like this:

validates :case_id,
        uniqueness: {
                      scope: :user_id,
                      message: 'Message'
                    },
        unless: Proc.new { |c| c.user_id.nil? }

The test I have for it, thus far, looks like this:

it { should validate_uniqueness_of(:case_id)
              .scoped_to(:user_id)
              .with_message('Message.') }

The problems is I don't know how to incorporate the unless conditional into the test?

Any ideas?