ci/kernel: Make download-prebuilt-kernel safe for set -u

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31281>

.gitlab-ci/container/download-prebuilt-kernel.sh

@@ -2,7 +2,7 @@
 # shellcheck disable=SC2086 # we want word splitting
 # shellcheck disable=SC2153
 
-set -ex
+set -uex
 
 mkdir -p kernel
 pushd kernel
@@ -11,12 +11,12 @@ if [[ ${DEBIAN_ARCH} = "arm64" ]]; then
     KERNEL_IMAGE_NAME+=" cheza-kernel"
 fi
 
-for image in ${KERNEL_IMAGE_NAME}; do
+for image in ${KERNEL_IMAGE_NAME:-}; do
     curl -L --retry 4 -f --retry-all-errors --retry-delay 60 \
       -o "/lava-files/${image}" "${KERNEL_IMAGE_BASE}/${DEBIAN_ARCH}/${image}"
 done
 
-for dtb in ${DEVICE_TREES}; do
+for dtb in ${DEVICE_TREES:-}; do
     curl -L --retry 4 -f --retry-all-errors --retry-delay 60 \
       -o "/lava-files/${dtb}" "${KERNEL_IMAGE_BASE}/${DEBIAN_ARCH}/${dtb}"
   done