Fixed export to not include 'stock' sets. Reset PCII number.

[randywoods]

🗣 Description

The export was always including a "stock" SET in the export file as if it was a CustomStandard. This was confusing the importer, which was trying to reconstitute a set that did not have enough detail.

We also null out the PCII number if they turn the PCII switch off. This prevents us from exporting an assessment that says PCII is false, but here's the PCII Number.

💭 Motivation and context

🧪 Testing

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.