fix SC2015 for CI shellcheck 0.9 (unambiguous if-forms)

CI red fix per the all-CI-green mandate; same pattern as the KNELCa/
KNELInventory fixes. No behavior change.
CI evidence: https://git.knownelement.com/KNEL/KNELPerf/actions
Ticket: https://projects.knownelement.com/issues/784

loop/baseline-diff.sh

@@ -17,7 +17,7 @@ join_tsv() { awk -F'\t' 'NR==FNR{a[$1]=$2;next} {print $1"\t"a[$1]"\t"$2"\t"$3}'
 # shellcheck disable=SC2034  # exit-status reserved for future strictness
 status=0
 join_tsv "$A/cpu-bench.tsv" "$B/cpu-bench.tsv" | while IFS=$'\t' read -r metric va vb unit; do
-  [ -n "$va" ] && [ -n "$vb" ] || continue
+  if [ -z "$va" ] || [ -z "$vb" ]; then continue; fi
   if [ "$va" = "$vb" ]; then
     printf 'OK    %-28s %s = %s %s\n' "$metric" "$va" "$vb" "$unit"
   elif [ "$va" = "${va#[-+0-9.]}" ] || [ "$vb" = "${vb#[-+0-9.]}" ]; then