kvserver: use actual empty proposal to unquiesce

[tbg]

We used to unquiesce via a "noop" but not nil log entry but it turns out that
it can happen that raft is out of budget for nontrivial log entries when
unquiescing. So, use a nil one which is identical to what raft proposes when
leadership changes.

Closes #100096.

Epic: none
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[tbg]

There actually is a probable downside to this PR, which is that we're interpreting the true empty entry as a signal that a new leader has stepped up, and react to applying it by unconditionally reproposing everything in flight at that point:

cockroach/pkg/kv/kvserver/replica_raft.go

Lines 1019 to 1025 in 4180659

	if stats.apply.numEmptyEntries > 0 {
	// Overwrite unconditionally since this is the most aggressive
	// reproposal mode.
	if !r.store.TestingKnobs().DisableRefreshReasonNewLeaderOrConfigChange {
	refreshReason = reasonNewLeaderOrConfigChange
	}
	}

So when a quiescent range sees multiple writes at once, we'll repropose some of these, pointlessly. That isn't great.

We'll have to mull this over a bit more, I wouldn't land this as-is.