Ouputformat - Fixing adding an OF

It now works when the default output format exists. I think we have to
create the default output formats at new mapfile initialization.

* tested with wolrd_mapfile [ok]
* with completely blank-new mf [ko]

Because a blank new mapfile does not have even 1 outputformat defined,
it cannot have a one by default, bypassing the OUTPUTFORMAT block
generation when saving the mapfile.