Fix followers synchronization mechanism not working when URI has empty path (#16744)

Follow-up to #16510, forgot the controller exposing the actual followers…

app/controllers/activitypub/followers_synchronizations_controller.rb

@@ -19,11 +19,11 @@ class ActivityPub::FollowersSynchronizationsController < ActivityPub::BaseContro
   private
 
   def uri_prefix
-    signed_request_account.uri[/http(s?):\/\/[^\/]+\//]
+    signed_request_account.uri[Account::URL_PREFIX_RE]
   end
 
   def set_items
-    @items = @account.followers.where(Account.arel_table[:uri].matches(uri_prefix + '%', false, true)).pluck(:uri)
+    @items = @account.followers.where(Account.arel_table[:uri].matches("#{Account.sanitize_sql_like(uri_prefix)}/%", false, true)).or(@account.followers.where(uri: uri_prefix)).pluck(:uri)
   end
 
   def collection_presenter

spec/controllers/activitypub/followers_synchronizations_controller_spec.rb

@@ -5,11 +5,13 @@ RSpec.describe ActivityPub::FollowersSynchronizationsController, type: :controll
   let!(:follower_1) { Fabricate(:account, domain: 'example.com', uri: 'https://example.com/users/a') }
   let!(:follower_2) { Fabricate(:account, domain: 'example.com', uri: 'https://example.com/users/b') }
   let!(:follower_3) { Fabricate(:account, domain: 'foo.com', uri: 'https://foo.com/users/a') }
+  let!(:follower_4) { Fabricate(:account, username: 'instance-actor', domain: 'example.com', uri: 'https://example.com') }
 
   before do
     follower_1.follow!(account)
     follower_2.follow!(account)
     follower_3.follow!(account)
+    follower_4.follow!(account)
   end
 
   before do
@@ -45,7 +47,7 @@ RSpec.describe ActivityPub::FollowersSynchronizationsController, type: :controll
 
       it 'returns orderedItems with followers from example.com' do
         expect(body[:orderedItems]).to be_an Array
-        expect(body[:orderedItems].sort).to eq [follower_1.uri, follower_2.uri]
+        expect(body[:orderedItems].sort).to eq [follower_4.uri, follower_1.uri, follower_2.uri]
       end
 
       it 'returns private Cache-Control header' do