Blocklog: Return std::optional for head_id

[heifner]

Minor refactor to make code cleaner/clearer. Return empty optional when block log does not have a current head.

Resolves #1289

[greg7mdp]

Upon reflection, it seems that we either have both { head, head_id }, or they are both null.

Maybe the optional should be std::optional<signed_block_ptr, block_id_type> head;, so it is clear that either both are present or none.

[heifner]

Upon reflection, it seems that we either have both { head, head_id }, or they are both null.

Maybe the optional should be std::optional<signed_block_ptr, block_id_type> head;, so it is clear that either both are present or none.

That does make it clearer. Done.