"stats: don't dump empty or zero counters" (2decb9ea) broke stat output in 3.3.0

Downstream bug: https://bugs.debian.org/1051067

In 3.3.0, stats are

---
time: 2023-09-01T23:03:44+0200
identity: szarotka.nabijaczleweli.xyz
server:
 zone-count: 25
mod-cookies:
 presence: 780
 mod-stats:
  request-protocol:
   udp4: 3776
   tcp4: 142
  server-operation:
   query: 3838
   axfr: 4
   ixfr: 76
  request-bytes:
   query: 208595
   other: 11080
  response-bytes:
   reply: 514727
   transfer: 13348
  response-code:
   NOERROR: 2755
   FORMERR: 1
   NXDOMAIN: 831
   REFUSED: 20
   NOTAUTH: 1
   BADCOOKIE: 310

This is naturally wrong, but just compare to 2.2.x:

---
time: 2023-08-29T00:19:44+0200
identity: szarotka.nabijaczleweli.xyz
server:
 zone-count: 24
mod-cookies:
 presence: 24
 dropped: 0
mod-rrl:
 slipped: 0
 dropped: 0
mod-stats:
 request-protocol:
  udp4: 24
 server-operation:
  query: 24
 request-bytes:
  query: 1632
 response-bytes:
  reply: 2292
 response-code:
  NOERROR: 12
  BADCOOKIE: 12

(how is mod-stats part of mod-cookies? what happened to mod-rrl)?

I rebuilt Debian 3.3.0-2 with reversed 2decb9ea, and got

---
time: 2023-09-02T00:34:56+0200
identity: szarotka.nabijaczleweli.xyz
server:
 zone-count: 25
mod-cookies:
 presence: 2
 dropped: 0
mod-rrl:
 slipped: 0
 dropped: 0
mod-stats:
 request-protocol:
  udp4: 4
 server-operation:
  query: 4
 request-bytes:
  query: 224
 response-bytes:
  reply: 395
 response-code:
  NOERROR: 3
  BADCOOKIE: 1

again. Please revert this.