rubocop_canvas: fix add_reference non-transactional logic

we don't need to suggest `disable_ddl_transaction!` when adding
a non-indexed reference

fixes FOO-3298
test plan: specs

Change-Id: I33ca47954e5760bf3f14d8b240891713f735eeda
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/308301
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

gems/rubocop-canvas/lib/rubocop_canvas/cops/migration/add_index.rb

@@ -104,13 +104,14 @@ module RuboCop
         end
 
         def check_add_reference(node)
-          check_non_transactional
-
           arg = index_argument(node).first
-          if arg.nil? || (!false?(arg) && !algorithm_concurrently?(arg))
+          if arg.nil? || !false?(arg)
+            check_non_transactional
+            if arg.nil? || !algorithm_concurrently?(arg)
               add_offense arg || node, message: INDEX_ALGORITHM_CONCURRENTLY_MSG, severity: :warning
             end
           end
+        end
 
         def check_change_table(node)
           ct_index(node).each { |subnode| check_add_index(subnode) }

gems/rubocop-canvas/spec/rubocop/cop/migration/add_index_spec.rb

@@ -145,7 +145,6 @@ describe RuboCop::Cop::Migration::AddIndex do
     it "doesn't complain if not indexed" do
       inspect_source(<<~RUBY)
         class TestMigration < ActiveRecord::Migration
-          disable_ddl_transaction!
           def up
             add_reference :users, :organizations, index: false
           end