Avoid using linear queries when traversal_ids is blank

app/models/namespaces/traversal/linear.rb

@@ -52,7 +52,9 @@ module Namespaces
       end
 
       def use_traversal_ids?
-        Feature.enabled?(:use_traversal_ids, root_ancestor, default_enabled: :yaml)
+        return false unless Feature.enabled?(:use_traversal_ids, root_ancestor, default_enabled: :yaml)
+
+        traversal_ids.present?
       end
 
       def self_and_descendants

spec/lib/gitlab/object_hierarchy_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::ObjectHierarchy do
-  let_it_be(:parent) { create(:group) }
+  let_it_be(:parent, reload: true) { create(:group) }
   let_it_be(:child1) { create(:group, parent: parent) }
   let_it_be(:child2) { create(:group, parent: child1) }
 

spec/models/namespace_spec.rb

@@ -880,7 +880,7 @@ RSpec.describe Namespace do
   end
 
   describe '#use_traversal_ids?' do
-    let_it_be(:namespace) { build(:namespace) }
+    let_it_be(:namespace, reload: true) { create(:namespace) }
 
     subject { namespace.use_traversal_ids? }
 
@@ -902,6 +902,8 @@ RSpec.describe Namespace do
   end
 
   context 'when use_traversal_ids feature flag is true' do
+    let_it_be(:namespace, reload: true) { create(:namespace) }
+
     it_behaves_like 'namespace traversal'
 
     describe '#self_and_descendants' do