`BlockId::root` doesn't work at `oldest_block_slot`

[michaelsproul]

While doing some testing I noticed that we're unable to load the block at the oldest_block_slot by slot:

curl -s "http://localhost:5052/eth/v1/beacon/blinded_blocks/3174625" | jq
{
  "code": 404,
  "message": "NOT_FOUND: beacon block at slot 3174625",
  "stacktraces": []
}

This is because block_root_at_slot_skips_none always tries to load the block root of the previous slot to the one queried, which fails when requesting the oldest_block_slot.

lighthouse/beacon_node/beacon_chain/src/beacon_chain.rs

Lines 1036 to 1047 in 999b045

	if let Some(((prev_root, _), (curr_root, curr_slot))) = process_results(
	self.forwards_iter_block_roots_until(prev_slot, request_slot)?,
	|iter| iter.tuple_windows().next(),
	)? {
	// Sanity check.
	if curr_slot != request_slot {
	return Err(Error::InconsistentForwardsIter {
	request_slot,
	slot: curr_slot,
	});
	}
	Ok((curr_root != prev_root).then_some(curr_root))

[jimmygchen]

Closed in #7693