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

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[codecov]

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