Enable creation of empty seedlots

[afpowell]

Description

This PR allows users to create empty seedlots as placeholders (i.e., seedlots with no/zero seed weight values or seed count values), as this seems desirable for certain users.

Closes #5157

Checklist

• [ ] Refactoring only
• [ ] Documentation only
• [ ] Fixture update only
• [ ] Bug fix
  • [ ] The relevant issue has been closed.
  • [ ] Further work is required.
• [ ] New feature
  • [ ] Relevant tests have been created and run.
  • [ ] Data was added to the fixture
    • [ ] Data was added via a patch in /t/data/fixture/patches/.
  • [ ] User-Facing Change
    • [ ] The user manual in /docs has been updated.
  • [ ] Any new Perl has been documented using perldoc.
  • [ ] Any new JavaScript has been documented using JSDoc.
  • [ ] Any new legacy JavaScript has been moved from /js to /js/source/legacy.