Changes arguments of DmsfLinksHelper#files_for_select

The second argument 'folder_id' is set to nil by default. This could
be overridden if necessary.

Defining a default value for the helper method prevents the view
partial 'dmsf_links/_form.html.erb' to raise an ActionView::Template::Error
due to a missing argument.

app/helpers/dmsf_links_helper.rb

@@ -35,7 +35,7 @@ module DmsfLinksHelper
     str&.match?(/\A\d+\Z/)
   end
 
-  def files_for_select(project_id, folder_id)
+  def files_for_select(project_id, folder_id = nil)
     files = []
     if DmsfLinksHelper.number?(folder_id)
       folder = DmsfFolder.find_by(id: folder_id)