catch case where n_eligible_for_doseX is 0, avoid division by 0

[edknock]

This catches the case where n_eligible_for_doseX is 0, where we want to avoid division by 0. I think I've got it right here that n_vaccination_t_X is just 0 in that case

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.03%. Comparing base (a47bb6e) to head (3309e74).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   60.03%   60.03%           
=======================================
  Files           9        9           
  Lines        2117     2117           
=======================================
  Hits         1271     1271           
  Misses        846      846           

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