add --empty flag to dbt build

[MichelleArk]

resolves #10026

Problem

dbt build does not support --empty. This was just an oversight in development.

We even document it as supported already: https://docs.getdbt.com/reference/commands/build#details

Solution

• Add --empty to the CLI options for dbt build
• Move adapter-zone tests to functional for test_empty. Adapters can maintain having overridable tests, but we should have a baseline test here for the flag that is more general.

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
• This PR includes type annotations for new and modified functions

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.20%. Comparing base (2d33655) to head (ab5ee72).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10027      +/-   ##
==========================================
+ Coverage   88.14%   88.20%   +0.06%     
==========================================
  Files         181      181              
  Lines       22625    22644      +19     
==========================================
+ Hits        19943    19974      +31     
+ Misses       2682     2670      -12     

Flag	Coverage Δ
integration	85.59% <100.00%> (+0.14%)	⬆️
unit	62.30% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.