sglang tp_dp_matrix: fix empty DATASET_PATH causing docker invalid spec

When DATASET_PATH is empty, -v ::ro is passed to docker run, which
produces 'invalid spec: ::ro: empty section between colons'. Build
the volume args dynamically and only add the dataset mount when the
path is non-empty.

experiments/dsv4_h200_sglang_tp_dp_matrix/run_bench.sh

@@ -120,11 +120,15 @@ restart_server() {
 run_bench_serving() {
   # Run sglang.bench_serving either natively or inside the SGLang Docker image.
   if [[ "${USE_DOCKER_CLIENT:-1}" == "1" ]]; then
+    local vol_args=()
+    vol_args+=("-v" "${MODEL_PATH}:${MODEL_PATH}:ro")
+    if [[ -n "${DATASET_PATH:-}" ]]; then
+      vol_args+=("-v" "${DATASET_PATH}:${DATASET_PATH}:ro")
+    fi
+    vol_args+=("-v" "${RESULT_BASE}:${RESULT_BASE}")
     docker run --rm \
       --network host \
-      -v "${MODEL_PATH}:${MODEL_PATH}:ro" \
-      -v "${DATASET_PATH}:${DATASET_PATH}:ro" \
-      -v "${RESULT_BASE}:${RESULT_BASE}" \
+      "${vol_args[@]}" \
       -e PYTHONUNBUFFERED=1 \
       "${DOCKER_IMAGE}" \
       python -m sglang.bench_serving "$@"