add tests for --empty flag

[MichelleArk]

resolves #949
docs dbt-labs/docs.getdbt.com/# N/A

Problem

dbt-labs/dbt-core#8971 introduced a new --empty flag that required implementation changes to the BaseRelation.

Solution

Add tests in dbt-bigquery to ensure expected behaviour of the new --empty flag when running with dbt-redshift.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

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