Remove divide by 0 warnings when running unit tests

[clmould]

Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.63%. Comparing base (25480ae) to head (7f2f46f).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4021      +/-   ##
==========================================
- Coverage   45.89%   42.63%   -3.27%     
==========================================
  Files         123      123              
  Lines       29064    32413    +3349     
==========================================
+ Hits        13340    13820     +480     
- Misses      15724    18593    +2869     

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[timothy-nunn]

Happy with the changes to tests and I'm 99% sure the changes to the model are fine but think a modeller should confirm it.

[timothy-nunn]

Can you request a modeller to review just this file and check they are happy with this bit. Maybe @chris-ashe ?