Sql id map assumes message table exists if map table exists

Problem/Motivation

The \Drupal\migrate\Plugin\migrate\id_map\Sql class assumes that the migration message table exists if the related map table exists.
This is OK in most cases, but in the context of ✨ Allow overriding id map migrate plugin Needs review , and using a custom id map plugin that allows overriding the map/message tables; the assumption does not hold up.

Proposed resolution

Remove the assumption, this can be even be only for correctness :-)

Remaining tasks

User interface changes

N.A.

API changes

N.A.

Data model changes

N.A.