In rec_write.c, we use F_ISSET(r, WT_REC_LOOKASIDE to decide whether to call __wt_las_wrapup. We also have r->cache_write_lookaside that tracks if we need lookaside writes for real.

Check that refs only end up with a las_pageid if they actually have lookaside entries. There is a check at the end of __wt_las_insert_block that suggests we can make it all the way through there and not write anything:

  549         if (insert_cnt > 0) {
  550                 (void)__wt_atomic_add64(
  551                     &S2C(session)->cache->las_entry_count, insert_cnt);
  552                 WT_ERR(__las_insert_block_verbose(session, multi));
  553         }