Fix unique key constraint in assets table

[guggero]

Currently the UNIQUE constraint in the assets is broken, since adding the primary key of the table to the UNIQUE constraint basically makes it useless: https://github.com/lightninglabs/taro-private/blob/main/tarodb/sqlc/migrations/000002_assets.up.sql#L177

AFAIK the constraint was changed to allow multiple assets with the NUMS script key.
Instead we could use NULL as the script_key_id, interpret that as being the NUMS point and turn the unique constraint into a CHECK function that checks different uniqueness levels, depending on whether the script_key_id is null or not.

[guggero]

Duplicate of #691.