Make check work with 'set -u'

bin/restic_backup.sh

@@ -38,11 +38,11 @@ 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
-	
+
 	if [ ${#unset_envs[@]} -gt 0 ]; then
 		printf "The following env variables are recommended, but have not been set. This script may not work as expected: %s\n" "${unset_envs[*]}" >&2
 	fi