During startup of iiod, we get the following warning:
iiod.service: Referenced but unset environment variable evaluates to an empty string: IIOD_EXTRA_OPTS

This is because since v254 of systemd, unset environmental variables will result in a warning (see commit).

I could make a PR where I add this to the service file:
Environment=IIOD_EXTRA_OPTS=''

Would this be a good solution?