defining global functions

Put them in a helper and include them in your controller(s).

If you want them to be available *everywhere*, write a new helper, say
GlobalHelper in a file called global_helper and in your
application_helper and application_controller say

class Application[Helper|Controller]
  include GlobalHelper

BTW, a better way to implement this function is:

def empty_string? s
  !(s && !s.empty?)

Better still, you can add an empty? method to NilClass, which negates
the need for the function in the first place:

class NilClass
  def empty?

This means that you can now call empty? on a nil object or a string,
nil always returning true.

If you have many functions like that, you should really spend some
time reading the pickaxe book and the Ruby api docs. Also have a look
at the ruby facets library. I think you'll find that a lot of what you
are trying to do is either already there or can be implemented in a
better way.