[CT-3310] Support --empty flag for dbt seed

[graciegoheen]

Housekeeping

• I am a maintainer of dbt-core

Short description

dbt seed --empty produces empty (0 row) seeds in the warehouse:

• on most DWHs this would be quite simple, since we already create empty table + then insert
• BQ implementation is quite different though

Acceptance criteria

dbt seed --empty produces empty (0 row) seeds in the warehouse

Impact to Other Teams

impact adapters

Will backports be required?

no

Context

blocked by #8980

[jtcohen6]

I'm moving issues from dbt-core to dbt-adapters that relate to implementation logic which now lives in the latter, but I'm leaving this one here because it's possible we could implement this within dbt-core

[dbeatty10]

Implementation hint

See #10962 for how this was implemented for snapshots.

[ChenyuLInx]

Potentially, put a limit here

[dbeatty10]

Potentially, put a limit here

Good idea! 💡

When empty is True for a seed, then we could try something like this:

        result.agate_table = agate_result.table.limit(0)

And otherwise leave it as-is:

        result.agate_table = agate_result.table