[CI] [Bugfix] Fix unbounded variable in `run-multi-node-test.sh` (#31967)

Signed-off-by: tjtanaa <tunjian.tan@embeddedllm.com>

.buildkite/scripts/run-multi-node-test.sh

@@ -7,7 +7,7 @@ set -euox pipefail
 if [ -e /dev/kfd ] || \
     [ -d /opt/rocm ] || \
     command -v rocm-smi &> /dev/null || \
-    [ -n "$ROCM_HOME" ]; then
+    [ -n "${ROCM_HOME:-}" ]; then
     IS_ROCM=1
 else
     IS_ROCM=0

.buildkite/test-pipeline.yaml

@@ -1104,6 +1104,7 @@ steps:
   - vllm/model_executor/models/
   - tests/distributed/
   - tests/examples/offline_inference/data_parallel.py
+  - .buildkite/scripts/run-multi-node-test.sh
   commands:
   - # the following commands are for the first node, with ip 192.168.10.10 (ray environment already set up)
     - VLLM_TEST_SAME_HOST=0 torchrun --nnodes 2 --nproc-per-node=2 --rdzv_backend=c10d --rdzv_endpoint=192.168.10.10 distributed/test_same_node.py | grep 'Same node test passed'