Enforce data tables for translatable entity types in the SQL entity storage

Problem/Motivation

Currently when generating the schema for a translatable entity type, we treat the data table as optional. The reason we introduced this exception was that when #2183231: Make ContentEntityDatabaseStorage generate static database schemas for content entities went in, not all content entity types were migrated to the standard table layout yet. Since core SQL entity storage is going to support dynamic switching between only 4 table layouts, we need to enforce the existence of the data table for translatable entity types. If we don't, besides needing to support more table layouts than what we need, "wrongly defined" entity types might have base fields not supporting translation even if their storage definitions say they do.

Proposed resolution

Remove that check on the existence of the data table when initializing the table layout and introduce a default value for the data table name when it is not defined in the entity type definition.

Remaining tasks

Reviews

User interface changes

None

API changes

None, this will just make things work as announced in the various change notices.