BuddyPress.org

I believe that it uses the built-in wordpress “post” function.

Why would you think that? Even a quick look at the code would show that’s not the case.

Use this hook:
apply_filters( 'bp_activity_post_update_content', $_POST['whats-new'] );

Try:

function allow_empty_whatsnew( $content ) {
	if ( empty( $content ) )
		$content = ' '; // cannot be empty
		
	return $content;
}
add_filter('bp_activity_post_update_content', 'allow_empty_whatsnew');