fix(cometbft) Prevent panic on non-nil proposal without vote extensions (backport #3270)

[mergify]

Description

We saw that gaiad has adoped cometbft v0.38 "HEAD" but we are aware of a panic bug affecting validator nodes.

We first experienced daemon panics on seda chain due to cometbft v0.38.6 calling panic when a non-nil proposal vote has vote extensions disabled.

The panic is a mistake and cometbft v0.38.11 includes our fix.

This panic is likely to affect gaiad regardless of vote extensions being enabled or not since the bug is:

extSignature = len([]byte{}) > 0 = false
precommit = true
isNil = false

if extSignature == (!precommit || isNil) => if faise == (!true || false) => if false == (false || false) => if false == false {
 panic()
}

Link to cometbft PR and discussion:
cometbft/cometbft#3565

This fix can reduce gaiad panics on upcoming mainnet v19

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This is an automatic backport of pull request #3270 done by [Mergify](https://mergify.com).

[mpoke]

Wait for v9.1.0 to be cut before merging this.

[mpoke]

closing as v19.1.0 already has this version of comet as a result of 92a2a88.