Fix mag and e_mag values when forcediffimflux and/or forcediffimfluxunc is -99999

[ale-munozarancibia]

Summary of the changes

Added conditions so that when forcediffimflux = -99999 (measurement could not be computed, as described in Masci+2023) then mag is nan, and similarly for forcediffimfluxunc and e_mag. These should naturally give mag_corr, e_mag_corr and e_mag_corr_ext as nan values too. Bug was described in #316.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (51d2328) 86.38% compared to head (97da122) 74.63%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #323       +/-   ##
===========================================
- Coverage   86.38%   74.63%   -11.76%     
===========================================
  Files          32        8       -24     
  Lines        1645      272     -1373     
===========================================
- Hits         1421      203     -1218     
+ Misses        224       69      -155     

Flag	Coverage Δ
unittest	74.63% <100.00%> (-11.76%)	⬇️

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