Tom Locke and I have similar proclivities. He says:

class Object
  def _?()
    self
  end
end

class NilClass
  def _?()
    SafeNil.new
  end
end

class SafeNil
  def method_missing(*args, &b)
    nil.send(*args, &b) rescue nil
  end
end

I say:

class Object
  def null
    n = nil
    def n.method_missing(*args)
      null
    end
    n
  end
end

The difference:

string_or_nil._?.length        # Tom's
(string_or_nil||null).length   # mine

I like being able to go more than one method deep,

(string_or_nil||null).length * 4

But that might not be to your taste.