Set value to initial value if None

[JamesOHeaDLS]

total_flux came back as None during testing

This change ensures the update function can handle this case

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.93%. Comparing base (c3e7900) to head (a12813e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   84.86%   84.93%   +0.07%     
==========================================
  Files          13       13              
  Lines         403      405       +2     
==========================================
+ Hits          342      344       +2     
  Misses         61       61              

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