Dal/Scoru: fix the way `ancestor_hash` works around origination_level

Context

The function should not return None for blocks/DAL slots published before a scoru's origination level. Otherwise, some invariants about how Dal skip list is initialized in the node's store is broken.

In fact, if slots_info returns None for slots published before origination_level, Confirmed_slots_history.update will not be called and Confirmed_slots_history table will not be (correctly) initialized. This results in an error:

"The confirmed DAL slots history for block hash %a (level = %a) is missing."

When processing the block of the next level.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
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