Fix SimpleOrderBuilder undefined method error

This change fixes the undefined method error when an AR scope is passed
with complex arel expression (CASE).

ee/spec/lib/gitlab/pagination/keyset/simple_order_builder_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Pagination::Keyset::SimpleOrderBuilder do
+  context 'when ordering by a CASE expression and id' do
+    let(:scope) { Vulnerability.order_report_type_asc }
+
+    subject(:result) { described_class.build(scope) }
+
+    it 'does not raise error' do
+      expect { result }.not_to raise_error
+    end
+
+    it 'does not support this ordering' do
+      _, success = result
+
+      expect(success).to eq(false)
+    end
+  end
+end

lib/gitlab/pagination/keyset/simple_order_builder.rb

@@ -122,6 +122,7 @@ module Gitlab
 
           return unless attribute
           return unless tie_breaker_attribute
+          return unless attribute.respond_to?(:name)
 
           model_class.column_names.include?(attribute.name.to_s) &&
             arel_table[primary_key].to_s == tie_breaker_attribute.to_s