[Feature]: Handle vacancies more cleanly

### Description

Ideally, we'd want to have something that identifies if a post is actually vacant. Currently if no matching member_id is found in the parliamentary API, then the code assumes the position is vacant. What I'd like to see is if we can more explicitly handle situations where there's no sitting MP.

### Use case

Trust how the code handles vacancies.

### Why this is important

Reliability.

### Additional Context

Conversation that was the source of this on #20. We should look into the parliamentary API more to understand our options here.

> ...if anything they might just falsely cover up NAs if they were to appear for any other reason
>
> Yeah, agreed. I was wary of that looking at how we handle the member info, but didn't want to change that as part of this PR. It would be useful to see if there's anything in the parliamentary API that flags if it is actually vacant, such that we can pull that info through properly.