Use get_adjacent_post to first see if there’s actually a next post:
if (get_adjacent_post (false, '', false)) {
next_post_link ('%link', 'Next Project >', true, '', 'projectcategory');
} else {
// manually create link to first post here
}
This site shows one way of actually getting the first post’s link, such as with a WP_Query call and then calling get_permalink. The exact code depends on the types of post you are showing.