Invoice number generation is broken

(Follow-up of http://drupal.org/node/1728106#comment-6997826)

There's a major design flaw in the way the callback-based method is implemented :

The 'switch/case' statement that give the callback a chance to be called actually resides in an if statement conditioned by the existence of some past invoices.

As a result the callback is never called if no invoice exists.

Here's a patch that move the callback logic outside this statement as the check of past invoice is in the scope of the numbering strategy itself, IMHO.