ci/va: Make build-va-tools 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/build-va-tools.sh

@@ -4,7 +4,7 @@
 # .gitlab-ci/image-tags.yml tags:
 # KERNEL_ROOTFS_TAG
 
-set -ex
+set -uex
 
 git config --global user.email "mesa@example.com"
 git config --global user.name "Mesa CI"
@@ -19,7 +19,7 @@ pushd /va-utils
 # Too old libva in Debian 11. TODO: when this PR gets in, refer to the patch.
 curl -L https://github.com/intel/libva-utils/pull/329.patch | git am
 
-meson setup build -D tests=true -Dprefix=/va $EXTRA_MESON_ARGS
+meson setup build -D tests=true -Dprefix=/va ${EXTRA_MESON_ARGS:-}
 meson install -C build
 popd
 rm -rf /va-utils