release-24.1: apply: fix initial snapshot assertion

[blathers-crl]

Backport 1/1 commits from #131503 on behalf of @pav-kv.

/cc @cockroachdb/release

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The broad assertion here is that all the entries composing a snapshot must have been applied by someone before (at least the sender of the snapshot has done it, otherwise it couldn’t have sent this snapshot).

And then there is an additional assertion (which failed occasionally before this PR) that there can’t be a snapshot without at least one entry applied. This is generally true, except that there are entries not registered by the "apply" stack: particularly, empty entries that raft leader appends at the start of its term. There is a comment to this extent in the asserter.go code:

cockroach/pkg/kv/kvserver/apply/asserter.go

Lines 254 to 259 in 5b0371b

	// INVARIANT: all commands have a command ID. etcd/raft may commit noop
	// proposals on leader changes that do not have a command ID, but we skip
	// these during application.
	if len(cmdID) == 0 {
	fail("applied command without ID")
	}

This commit modifies the snapshot assertion to correctly handle this case. It is possible that an initial snapshot sent from leader to followers contains only this dummy entry, and there were no "real" proposals applied.

Fixes the false assertion failure in #118471 (comment)
Related to #116319

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Release justification: test deflake

[blathers-crl]

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[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?

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[cockroach-teamcity]

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