Update block_log and fork_db to return empty std::optional when requested data not present

[greg7mdp]

We can have empty block_logs and fork_db, and when that happens APIs like fork_db.root() and blog.first_block_num() make up credible values that are not always consistent. For example, in controller.cpp::log_irreversible(), we retrieve the blog head with:

block_id_type log_head_id = blog.head_id(); which always return a sha256, and the we check if it is valid by checking if it is all zeros.

And if it is invalid, then we check that fork_db.root()->block_num == blog.first_block_num() - 1.

It seems to me that if we had blog.head_id() returning an empty std::optional to mean that the blog is empty, then maybe the only think we would need to check in the assert is that fork_db.root() also return std::nullopt.

PS: If I messed up the explanation, Kevin understands what I mean and is welcome to fix it.

[heifner]

The odd invariant we are currently maintaining is first_block_num() returning a block number for a block that is actually not in the block log. This clearly looks like a hack.