Fix a reference to an undefined variable in a script that's set -u.

debian/changelog

@@ -26,6 +26,7 @@ livecd-rootfs (2.408.19) UNRELEASED; urgency=medium
   * If we're using SUBPROJECT=minimize, and tzdata is not installed, remove
     files that have been left behind.  This is a workaround for a bug that
     should be fixed in tzdata.
+  * Fix a reference to an undefined variable in a script that's set -u.
 
   [ Balint Reczey ]
   * Mount using --make-rslave to ensure safe unmounts for rbind mounts

live-build/ubuntu-cpc/hooks/033-disk-image-uefi.binary

@@ -61,7 +61,7 @@ install_grub() {
     efi_boot_dir="/boot/efi/EFI/BOOT"
     chroot mountpoint mkdir -p "${efi_boot_dir}"
 
-    if [ "$SUBPROJECT" = minimize ] && [ -n "$partuuid" ]; then
+    if [ "${SUBPROJECT:-}" = minimize ] && [ -n "$partuuid" ]; then
         # FIXME: code duplicated between 032-disk-image.binary
         # and 033-disk-image-uefi.binary.  We want to fix this to not
         # have initramfs-tools installed at all on these images.