system_syncState: Always return highest block (#11979)

Before `highestBlock` was an optional that was omitted when it was `None`. We recently changed the
way the `highestBlock` is determined, this resulted in having this value in 99.99% of the time being
`None` when the node is syncing blocks at the tip. Now we always return a block for `highestBlock`.
If sync doesn't return us any best seen block, we return our own local best block as `highestBlock`.
This should mainly reflect the same behavior to before we changed the way the best seen block is determined.