Fix calculate_age_at_registration

[pavlo-mk]

Fix calculate_age_at_registration

Usually we are using calculate_age_at_registration to calculate age_at_registration and field is age_at_registration = models.PositiveSmallIntegerField(null=True, blank=True, help_text="Age at registration [sys]") so we can have any values below 0

AB#309211: Angola: Kobo issue "Import Error" - example was this issue with kobo import

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (abf642f) to head (4ed7d4d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5883   +/-   ##
========================================
  Coverage    92.25%   92.26%           
========================================
  Files          494      494           
  Lines        34349    34354    +5     
  Branches      3602     3603    +1     
========================================
+ Hits         31688    31695    +7     
+ Misses        1911     1909    -2     
  Partials       750      750           

Flag	Coverage Δ
e2e	52.64% <14.28%> (-0.01%)	⬇️
unit	91.96% <100.00%> (+<0.01%)	⬆️

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