fix(behavior_velocity_planner): fix invalid access (#3187)

Signed-off-by: Takayuki Murooka <takayuki5168@gmail.com>

Author: takayuki5168

planning/behavior_velocity_planner/src/scene_module/intersection/scene_intersection.cpp

@@ -221,7 +221,8 @@ bool IntersectionModule::modifyPathVelocity(PathWithLaneId * path, StopReason *
 
   /* calculate final stop lines */
   std::optional<size_t> stop_line_idx =
-    std::make_optional<size_t>(stop_lines_idx_opt.value().collision_stop_line);
+    stop_lines_idx_opt ? std::make_optional<size_t>(stop_lines_idx_opt.value().collision_stop_line)
+                       : std::nullopt;
   if (external_go) {
     is_entry_prohibited = false;
   } else if (external_stop) {