Do not to_s the machine id if it is nil

When destroying a machine, the machine id is set to nil, the to_s causes it to be set to empty string.  This can cause inconsistent behavior in anything (such as plugins) that tests the machine id.

lib/vagrant/machine.rb

@@ -230,7 +230,7 @@ module Vagrant
       end
 
       # Store the ID locally
-      @id = value.to_s
+      @id = value.nil? ? nil : value.to_s
 
       # Notify the provider that the ID changed in case it needs to do
       # any accounting from it.