kvserver: allow empty ranges to be merged regardless of QPS

I noticed while running some ORM tests that empty ranges never got merged
away. When I manually range them through the queue, I got the report:

```
skipping merge: LHS QPS measurement not yet reliable
```

All of the ranges in question were totally empty. This small
patch seems to fix the problem.

Release note (bug fix): Empty ranges sometimes will now be merged away
regardless of QPS data.

pkg/kv/kvserver/merge_queue.go

@@ -252,12 +252,13 @@ func (mq *mergeQueue) process(
  // merges, the mergeQueue will only consider a merge when it deems the
  // maximum qps measurement from both sides to be sufficiently stable and
  // reliable, meaning that it was a maximum measurement over some extended
- // period of time.
- if !lhsQPSOK {
+ // period of time. However, if the range is empty, we don't care about
+ // the QPS.
+ if !lhsQPSOK && lhsStats.LiveBytes > 0 {
  log.VEventf(ctx, 2, "skipping merge: LHS QPS measurement not yet reliable")
  return false, nil
  }
- if !rhsQPSOK {
+ if !rhsQPSOK && rhsStats.LiveBytes > 0 {
  log.VEventf(ctx, 2, "skipping merge: RHS QPS measurement not yet reliable")
  return false, nil
  }