Fix run_webots.sh exit-1 when N=0 (calibration mode)

`active=$(grep -c '^Sheep' "$DST")` returns 0 with exit code 1 when
no sheep are left in the world, which fires set -e and kills the
script before it can launch Webots. Wrap with `|| true` so the
calibration mode (N=0) can actually run.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/run_webots.sh

@@ -115,7 +115,7 @@ for i in $(seq $((N+1)) 10); do
     sed -i "s|^Sheep .* \"sheep${i}\".*|# &|" "$DST"
 done
 
-active=$(grep -c '^Sheep' "$DST")
+active=$(grep -c '^Sheep' "$DST" || true)
 echo "------------------------------------------------------------"
 echo "World      : $DST"
 echo "Mode       : $MODE"