0.17.0: Cannot execute an empty query

[drewbanin]

Describe the bug

dbt is errorring with cannot execute and empty query for a model that does not have any columns described in a schema.yml file, but does have persist_docs: {columns: true} configured.

Steps To Reproduce

1. Create a simple model
2. Enable column-level docs persistence
3. Do not add column descriptions for the model
4. Run dbt run and observe the error

Expected behavior

I would expect dbt to not run a query, rather than running an empty query which fails!

System information

I replicated this on Postgres, but I imagine it could impact other databases too.

The output of dbt --version:

0.17.0-rc1