Prevent a 500 when a repository's root_ref is nil

Closes #14259

app/views/layouts/header/_default.html.haml

@@ -46,6 +46,8 @@
       %h1.title= title
 
 = render 'shared/outdated_browser'
+
 - if @project && !@project.empty_repo?
+  - if ref = @ref || @project.repository.root_ref
     :javascript
-    var findFileURL = "#{namespace_project_find_file_path(@project.namespace, @project, @ref || @project.repository.root_ref)}";
+      var findFileURL = "#{namespace_project_find_file_path(@project.namespace, @project, ref)}";