flags.THREADS defaults to None

[MichelleArk]

resolves: N/A

Currently, threads defaults to 1 and is overriding the profile-configured threads value (because it's not None by default!)

Updates the default for --threads to None so that it only overrides the profile-configured threads value if explicitly set.

From the documentation: https://docs.getdbt.com/docs/get-started/connection-profiles#understanding-threads

You can use a different number of threads than the value defined in your target by using the --threads option when executing a dbt command.

Description

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue

🎩 threads: 4 set in profiles.yml:

python3 core/dbt/cli/main.py run 
22:07:05  Unable to do partial parsing because a project config has changed
22:07:06  Found 7 models, 21 tests, 0 snapshots, 0 analyses, 502 macros, 0 operations, 3 seed files, 0 sources, 0 exposures, 0 metrics
22:07:06  
22:07:08  Concurrency: 4 threads (target='dev')
...

threads: 4 set in profiles.yml, --threads 1:

❯ python3 core/dbt/cli/main.py run --threads 1
22:07:18  Found 7 models, 21 tests, 0 snapshots, 0 analyses, 502 macros, 0 operations, 3 seed files, 0 sources, 0 exposures, 0 metrics
22:07:18  
22:07:22  Concurrency: 1 threads (target='dev')

• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

cc @jtcohen6

[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.

[ChenyuLInx]

LGTM