fix: set-u-safe empty cb_arg expansion (cohorts without q_values.txt)

When CBaSE's qvals step is absent (no q_values.txt, e.g. CNS_Cancer where qvals
crashed but params succeeded), the empty cb_arg array expansion aborted the whole
pipeline under set -u, blocking DIG + MutSig identify. Use ${cb_arg[@]+...} guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ahmed-shuaibi

scripts/run_cohort_pipeline.sh

@@ -52,7 +52,8 @@ if [ ! -f "${ROOT}/${C}/id_cbase/pairwise_interaction_results.csv" ] \
   mkdir -p "${ROOT}/${C}/id_cbase"
   cb_arg=(); [ -f "$CB_Q" ] && cb_arg=(-cb "$CB_Q")
   "$DIALECT" identify -c "${ROOT}/${C}/count_matrix.csv" -b "${ROOT}/${C}/bmr_pmfs.csv" \
-    -o "${ROOT}/${C}/id_cbase" -k 100 "${cb_arg[@]}" >>"$LOGF" 2>&1 || log "STAGE-FAIL id_cbase"
+    -o "${ROOT}/${C}/id_cbase" -k 100 "${cb_arg[@]+"${cb_arg[@]}"}" >>"$LOGF" 2>&1 \
+    || log "STAGE-FAIL id_cbase"
 fi
 if [ ! -f "${ROOT}/${C}/id_dig/pairwise_interaction_results.csv" ] \
    && [ -f "${ROOT}/${C}/bmr_pmfs.dig.csv" ]; then