interop: Handle ErrParentToFirst from PreviousDerivedFrom

[axelKingsley]

Prior PRs changed the behavior of WithParent and PreviousDerivedFrom to not panic, and instead error, when the PreviousDerivedFrom can't be determined.

However, the codepaths which use this are expected to function even on the first row of the database. Functions like CrossDerivedFromBlockRef and CanddiateCrossSafe need to handle the fact that there is no parent item for the DerivedFrom in question.

This PR adds the same basic handling -- if the error matches ErrPreviousToFirst, the calling functions use ForceWithParent(BlockRef{}) instead of erroring or panicing.

The devnet is currently running, but we see a lot of

t=2024-11-05T22:16:57+0000 lvl=warn msg="Failed to process work" chain=11473209 err="failed to determine candidate block for cross-safe: failed to find parent-block of cross-derived-from BlockSeal(hash:0xa39fd1a98c35ba3bc981366b87def30b7e796896bdc52988e4c1008524c415ae, number:7012090, time:1730748744): cannot find previous derived before start of database: 0xa39fd1a98c35ba3bc981366b87def30b7e796896bdc52988e4c1008524c415ae:7012090"

in the logs. This is because the workflows which expect to be function on the first row of the database are returning errors inappropriately.