Chore: add default for unwrap

[vigith]

If we do not add a default, I am getting a runtime panic because leader-id could be None?

thread 'main' panicked at src/pb_impl/impl_vote.rs:30:37:
called `Option::unwrap()` on a `None` value
stack backtrace:

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

Thank you! I missed this default value handling!

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @vigith)