Make check work with 'set -u'

bin/restic_backup.sh

@@ -38,7 +38,7 @@ warn_on_missing_envvars() {
 	local unset_envs=()
 	local varnames=("$@")
 	for varname in "${varnames[@]}"; do
-		if [ -z "${!varname}" ]; then
+		if [ -z "${!varname-}" ]; then
 			unset_envs=("${unset_envs[@]}" "$varname")
 		fi
 	done