Fix 404 page when viewing TODOs that contain milestones or labels in different projects

A user viewing the TODOs page will see a 404 if there are mentioned labels
in multiple different projects. This is likely a caching bug and only occurs
when Markdown rendering occurs across multiple projects, which is why it's so
tricky to reproduce. This is what I think is happening:

1. LabelReferenceFilter#references_in encounters label ~X for ProjectA and finds the label in the DB as id = 1.
2. LabelReferenceFilter.references_in yields [1, 'X', nil, ...]
3. Since project_ref is nil, AbstractReferenceFilter#project_from_ref_cache caches nil => ProjectA.
4. LabelReferenceFilter#references_in encounters label ~Y for ProjectB and finds the label in the DB as id = 2.
5. LabelReferenceFilter.references_in yields [2, 'Y', nil, ...]
6. AbstractReferenceFilter#project_from_ref_cache lookups nil and returns ProjectA. It was supposed to be ProjectB.
7. A is the wrong project, so the label lookup fails.

This MR caches Markdown references if the key is present.

Closes #17898

CHANGELOG

@@ -6,6 +6,7 @@ v 8.9.0 (unreleased)
   - Allow forking projects with restricted visibility level
   - Improve note validation to prevent errors when creating invalid note via API
   - Remove project notification settings associated with deleted projects
+  - Fix 404 page when viewing TODOs that contain milestones or labels in different projects
   - Redesign navigation for project pages
   - Fix groups API to list only user's accessible projects
   - Redesign account and email confirmation emails

lib/banzai/filter/abstract_reference_filter.rb

@@ -236,7 +236,9 @@ module Banzai
         if cache.key?(key)
           cache[key]
         else
-          cache[key] = yield
+          value = yield
+          cache[key] = value if key.present?
+          value
         end
       end
     end

spec/features/todos/todos_spec.rb

@@ -43,6 +43,27 @@ describe 'Dashboard Todos', feature: true do
       end
     end
 
+    context 'User has Todos with labels spanning multiple projects' do
+      before do
+        label1 = create(:label, project: project)
+        note1 = create(:note_on_issue, note: "Hello #{label1.to_reference(format: :name)}", noteable_id: issue.id, noteable_type: 'Issue', project: issue.project)
+        create(:todo, :mentioned, project: project, target: issue, user: user, note_id: note1.id)
+
+        project2 = create(:project)
+        label2 = create(:label, project: project2)
+        issue2 = create(:issue, project: project2)
+        note2 = create(:note_on_issue, note: "Test #{label2.to_reference(format: :name)}", noteable_id: issue2.id, noteable_type: 'Issue', project: project2)
+        create(:todo, :mentioned, project: project2, target: issue2, user: user, note_id: note2.id)
+
+        login_as(user)
+        visit dashboard_todos_path
+      end
+
+      it 'shows page with two Todos' do
+        expect(page).to have_selector('.todos-list .todo', count: 2)
+      end
+    end
+
     context 'User has multiple pages of Todos' do
       before do
         allow(Todo).to receive(:default_per_page).and_return(1)