release add page broken w/ cvs.module enabled and no compatibility taxonomy

if you:
- enable project_release.module
- enable cvs.module
- do *NOT* define any terms in the "Project release API compatibility" vocabulary
- try to create a HEAD release node

you get screwed by a bug where we're (presumably) assuming the compatibility taxomony is being used and refusing to advance to the next page without a value. i'm not surprised this bug exists, IIRC there are still a bunch of //TODO comments in the relevant code about "this doesn't work if you're not using the taxonomy stuff..." ;)

i noticed while testing the 5.x port (http://drupal.org/node/99759#comment-169234) but i just verified the bug is present in 4.7.x-2.1, too, so it's nothing broken with the 5.x port itself, it's a bug in the underlying code.