I've got Ruby 1.8.4 on OSX with many gems installed.
When I do
ri empty

I get this:

More than one method matched your request. You can refine
your search by asking for information on one of:

      Array#empty?, Hash#empty?, Queue#empty?, Set#empty?,  
String#empty?,
      StringScanner#empty?, ThreadsWait#empty?

Looks like you've confused a method from another class.

You might want to use something like this:

def dir_empty?(path_to_dir)
   Dir.chdir(path_to_dir)
   if Dir.glob('*').length > 0
    return false
   else
    return true
end