Set sto2 vars as nullable

[i-oden]

Before submitting this PR

1. Description: When Erik was trying to add a new unit to the testing instance, he got an error message saying that sto2* arguments could not be null. It appears that we never set them to nullable in the database when we added the sto4 variables. This PR does that.
2. Jira task / GitHub issue: DDS-1663
3. How to test: Add information on how someone could manually test this functionality. As detailed as possible.
4. Type of change: Check the relevant boxes in the section below
5. Add docstrings and comments to code, even if you personally think it's obvious.

What type of change(s) does the PR contain?

• [ ] New feature
  • [ ] Breaking: Please describe the reason for the break and how we can fix it.
  • [ ] Non-breaking
• [x] Database change
  • [x] Migration included in PR
  • [ ] Migration not needed
• [ ] Bug fix
  • [ ] Breaking: Please describe the reason for the break and how we can fix it.
  • [ ] Non-breaking
• [ ] Security Alert fix
• [ ] Documentation
• [ ] Tests (only)
• [ ] Workflow

Checklist

• Sprintlog
  • [x] Added
  • [ ] Not needed (E.g. PR contains only tests)
• Rebase / Update / Merge from base branch (the branch from which the current is forked)
  • [ ] Done
  • [x] Not needed
• Blocking PRs
  • [ ] Merged
  • [x] No blocking PRs
• PR to master branch
  • [ ] Yes: Read the release instructions
  • [ ] I have followed steps 1-8.
  • [x] No

Actions / Scans

• Black: Python code formatter. Does not execute. Only tests. Run black . locally to execute formatting.
  • [x] Passed
• Prettier: General code formatter. Our use case: MD and yaml mainly. Run npx prettier --write . locally to execute formatting.
  • [x] Passed
• Yamllint: Linting of yaml files.
  • [x] Passed
• Tests: Pytest to verify that functionality works as expected.
  • [x] New tests added
  • [ ] No new tests
  • [x] Passed
• CodeQL: Scan for security vulnerabilities, bugs, errors
  • [ ] New alerts: Go through them and either fix, dismiss och ignore. Add reasoning in items below.
  • [ ] Alerts fixed: What?
  • [ ] Alerts ignored / dismissed: Why?
  • [x] Passed
• Trivy: Security scanner
  • [ ] New alerts: Go through them and either fix, dismiss och ignore. Add reasoning in items below.
  • [ ] Alerts fixed: What?
  • [ ] Alerts ignored / dismissed: Why?
  • [x] Passed
• Snyk: Security scanner
  • [ ] New alerts: Go through them and either fix, dismiss och ignore. Add reasoning in items below.
  • [ ] Alerts fixed: What?
  • [ ] Alerts ignored / dismissed: Why?
  • [x] Passed

[codecov[bot]]

Codecov Report

Merging #1462 (e029629) into dev (190e114) will increase coverage by 0.54%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1462      +/-   ##
==========================================
+ Coverage   89.51%   90.05%   +0.54%     
==========================================
  Files          29       29              
  Lines        4396     4396              
==========================================
+ Hits         3935     3959      +24     
+ Misses        461      437      -24     

Files Changed	Coverage Δ
dds_web/database/models.py	96.18% <100.00%> (ø)

... and 1 file with indirect coverage changes