Default note *_access_level to DISABLED instead of nil

This relates to some more possible edge cases we may have in our data.
Since we are running a migration to backfill all these settings that
will run forever unless a value is set it is better to always have some
fallback that isn't `nil` as the migration will never finish.

In this case we can be sure that anything that is required to have a
project but doesn't for whatever reason shouldn't show up in searches
and therefore it's safe to just set `*_access_level = DISABLED`.

ee/lib/elastic/instance_proxy_util.rb

@@ -57,7 +57,7 @@ module Elastic
     # protect against missing project and project_feature and set visibility to PRIVATE
     # if the project_feature is missing on a project
     def safely_read_project_feature_for_elasticsearch(feature)
-      return unless target.project
+      return ProjectFeature::DISABLED unless target.project
 
       if target.project.project_feature
         target.project.project_feature.access_level(feature)

ee/spec/models/concerns/elastic/note_spec.rb

@@ -181,6 +181,19 @@ RSpec.describe Note, :elastic do
         expect(note_json).to have_key('visibility_level')
         expect(note_json['visibility_level']).to eq(expected_visibility_level)
       end
+
+      context 'when the project does not exist' do
+        before do
+          note.project = nil
+        end
+
+        it 'has DISABLED access_level' do
+          if access_level
+            expect(note_json).to have_key(access_level)
+            expect(note_json[access_level]).to eq(ProjectFeature::DISABLED)
+          end
+        end
+      end
     end
   end