release/vm: empty sh functions are disallowed

This happens to work with FreeBSD's shell, but is rejected by dash and
bash.

Reported by:	def
Reviewed by:	emaste
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D49526

release/tools/vmimage.subr

@@ -53,6 +53,7 @@ vm_create_base() {
 
 vm_copy_base() {
 	# Defunct
+	return 0
 }
 
 vm_install_base() {