Handle legacy repository archive requests with no ref given

The legacy endpoint requires no reference and defaults to the root
ref.

Closes #45154

app/controllers/projects/repositories_controller.rb

@@ -16,8 +16,10 @@ class Projects::RepositoriesController < Projects::ApplicationController
   def archive
     append_sha = params[:append_sha]
 
-    shortname = "#{@project.path}-#{@ref.tr('/', '-')}"
-    append_sha = false if @filename == shortname
+    if @ref
+      shortname = "#{@project.path}-#{@ref.tr('/', '-')}"
+      append_sha = false if @filename == shortname
+    end
 
     send_git_archive @repository, ref: @ref, format: params[:format], append_sha: append_sha
   rescue => ex
@@ -27,6 +29,9 @@ class Projects::RepositoriesController < Projects::ApplicationController
 
   def assign_archive_vars
     @id = params[:id]
+
+    return unless @id
+
     @ref, @filename = extract_ref(@id)
   rescue InvalidPathError
     render_404

spec/controllers/projects/repositories_controller_spec.rb

@@ -34,6 +34,12 @@ describe Projects::RepositoriesController do
         expect(response.header[Gitlab::Workhorse::SEND_DATA_HEADER]).to start_with("git-archive:")
       end
 
+      it 'handles legacy queries with no ref' do
+        get :archive, namespace_id: project.namespace, project_id: project, format: "zip"
+
+        expect(response.header[Gitlab::Workhorse::SEND_DATA_HEADER]).to start_with("git-archive:")
+      end
+
       context "when the service raises an error" do
         before do
           allow(Gitlab::Workhorse).to receive(:send_git_archive).and_raise("Archive failed")