No one else has offered anything up here. I thought I would let you know you're not alone trying to find the answer.
I have the same question and I'm starting to think varnishstat perhaps doesn't report metrics with entirely null values.
I have a test-bed server that I might deliberately starve of cache storage and see what happens. If I can confirm this behaviour, I will report back.
Perhaps someone might offer their expertise?
OK I can confirm that varnishstat appears to save terminal space by not reporting counters that are entirely 0 or null.
This is because of the continuously updated display mode.
If you run varnishstat non-interactive / continuous then the stat does indeed show.
[root@test nginx]# varnishstat -1 -f n_lru_nuked,n_expired,n_sess,n_sess_mem
n_sess_mem 20 . N struct sess_mem
n_sess 0 . N struct sess
n_expired 57 . N expired objects
n_lru_nuked 0 . N LRU nuked objects