Make db:check-schema run only if migrations have been added

This CI step is designed to make sure new migrations generate the proper changes in db/structure.sql, but it cannot detect unintended changes in db/structure.sql for already-introduced migrations. For it to do that, it would need to rewind the database before the migration were introduced, apply the migration, and then compare the resulting db/structure.sql.

To reduce false positives, we check if any migrations were added and only then do we allow scripts/regenerate-schema to run.

Closes #216519 (closed)