In the process to upgrade from 12.1-RELEASE-p13 to 12.2-RELEASE, the freebsd-update command fails during the configuration merge phase. The merged files proposed are all empty.
After looking at the code, it appears that the script calls the "merge" command, which doesn't exist on my system.
I've manually change the script to use diff3, which solved the issue (inspired from the latest version of freebsd-update on github using diff3).
ls on freebsd-update:
-r-xr-xr-x 1 root wheel - 90K Oct 12 2019 /usr/sbin/freebsd-update*
file version extracted from first lines:
# $FreeBSD: releng/11.3/usr.sbin/freebsd-update/freebsd-update.sh 345802 2019-04-02 13:58:31Z emaste $