Do not create snapshots for resources with no attributes

[nengru]

Hey maybe a suggestion, for a neat fail-safe.

If you use ash.codegen directly after ash.gen.resource and than ash.migrate, you create an empty migration file and a snapshot to a table without a table.
If you than add attributes and use ash.codegen + ash.migrate it will fail on drop and alter in the migration file because there is no table.
Did it a few times on accident, fixed the migration file manually, not sure if its a feature or ash.codegen should just abort it because it has no attributes or relationships

Best regards

[zachdaniel]

Thanks for the report. We should skip creating snapshots or migrations for tables w/ zero columns. PRs welcome!