dbt catalog doesn't support missing "database" fields

[beckjake]

Describe the feature

The dbt catalog functionality doesn't support missing database fields.

In a number of places, dbt expects database to be a str, but we should allow it to be None for some databases that don't have support for databases.

• Catalogs should be allowed to set the database to None for data warehouses that don't have them.
• list_relations should be allowed to take None for its database
  • on most data warehouses, None should either be an error or mean target.database.
  • on Spark, anything but None or the schema would be an error.
• get_relation should get the same treatment
• That's probably it?

Should this be extended to schemas?

Describe alternatives you've considered

We can set database='' everywhere. Kind of ugly! I would like to be able to differentiate "this component does not exist/has no meaning" vs "this is the empty string"...
Edit: This actually doesn't work without writing a bunch of new agate type tester things because by default, '' is coerced into None (which makes sense for seeds!).

Additional context

This is mostly relevant for Spark.

Who will this benefit?

Spark users, developers.

[beckjake]

Fixed in #2489