fix: Chain::split returns NoSplitPending for block number > tip

[fgimenez]

If the function split() is called with a block number greater than the tip it will consider this split valid and treat it the same as splitting the canonical head.

If the block_number > chain_tip then it implies the block is not in this chain and the chain cannot be split, with these changes a ChainSplit::NoSplitPending is returned (same as with missing hash).