fold-security-update-branches: Fix unbound variable

Fix error caused by unbound variable.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

release-scripts/fold-security-update-branches

@@ -257,7 +257,7 @@ case "$1" in
         ;;
     fold)
         FOLD=1
-        if [ -z "$2" ]
+        if [ -z "${2+x}" ]
         then
             echo "Folding to main repositories"
         else