Fix issues with processing toot edits (#17748)

* Fix searching for an already-known status by URL not working

* Fix Update processing from statuses prior to 20220302232632

`ordered_media_attachment_ids_changed?` would return `true` when going from
`nil` to anything (including `[]`).

* Add tests

spec/services/activitypub/process_status_update_service_spec.rb

@@ -46,6 +46,26 @@ RSpec.describe ActivityPub::ProcessStatusUpdateService, type: :service do
       expect(status.reload.spoiler_text).to eq 'Show more'
     end
 
+    context 'with no changes and originally with no ordered_media_attachment_ids' do
+      let(:payload) do
+        {
+          '@context': 'https://www.w3.org/ns/activitystreams',
+          id: 'foo',
+          type: 'Note',
+          content: 'Hello world',
+        }
+      end
+
+      before do
+        status.update(ordered_media_attachment_ids: nil)
+        subject.call(status, json)
+      end
+
+      it 'does not record an update' do
+        expect(status.reload.edited?).to be false
+      end
+    end
+
     context 'originally without tags' do
       before do
         subject.call(status, json)