Fix unbound variable errors

seqs/debianrpi.sh

@@ -39,7 +39,7 @@ step_1() {
   SDDEV="$2"
   if [ -z "$1" ] && interactive ; then
     read -p "Provide image download url from $toolDownSite: " toolDownUrl
-  elif [ ! -z "$1" ] ; then
+  elif [ -n "${1:-}" ] ; then
     toolDownUrl="$1"
   else
     error -e "No image file provided for download"
@@ -246,7 +246,7 @@ HDBOOT_EOF
 
 read_sd_dev() {
   local partExt=""
-  if [ ! -z "$1" ] ; then
+  if [ -n "${1:-}" ] ; then
     SDBOOT=
     SDROOT=
     SDROOTDEV=