"Do nothing" option doesn't work as expected

I think the first option on 'Update action' doesn't work as expected if a node doesn't have an alias.

How to reproduce:

1. Edit an existing node without an alias assigned
2. Ensure that 'Automatic alias' is checked on
3. Save node. Alias is not generated.

I believe that the problem is at pathauto.inc @ 257. Code checks update action, but there is no check about existing alias.

Please, check attached patch.