Warnings about undefined variables with systemd starting with version v254

Starting with v254 systemD services complain about unset variables, leading to

mariadb.service: Referenced but unset environment variable evaluates to an empty string: MYSQLD_OPTS, _WSREP_NEW_CLUSTER

warnings.

This was caused by this systemD changeset:

https://github.com/systemd/systemd-stable/commit/f331434d13488425ccd8485327085d15f8f92aea

The general recommendation to prevent this seems to be to just set such variables to an empty value up front in the service file, e.g.:

Environment=MYSQLD_OPTS=''

If a variable is then later set by an additional service configuration file, e.g.:

/etc/systemd/system/mariadb.service.d/override.conf

created via `systemctl edit mariadb.service`, then everything is fine and the custom value is used.

But when setting an environment variable via `systemctl set-environment`, as e.g. used by the `galera_new_cluster` script to set `_WSREP_NEW_CLUSTER`, this does NOT seem to work and an empty assignment in the `[Service]` section takes precedence over the value set via `systemctl set-environment`.

So right now I don't see a good way to set an environment variable conditionally within a systemD service file ...