GetOccurrences(): Fix NPE when returning a ToDo's occurrences that don't have a duration

[minichma]

TODO's don't necessarily have a duration, so the occurrences returned by GetOccurrences may return an EffectiveDuration of null. Such cases caused an NPE that is fixed with this PR.

fixes #824

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❌ Your project status has failed because the head coverage (68%) is below the target coverage (80%). You can increase the head coverage or adjust the target coverage.

@@         Coverage Diff         @@
##           main   #826   +/-   ##
===================================
  Coverage    68%    68%           
===================================
  Files       106    106           
  Lines      4209   4209           
  Branches    945    945           
===================================
  Hits       2842   2842           
  Misses     1039   1039           
  Partials    328    328           

Files with missing lines	Coverage Δ
Ical.Net/Evaluation/RecurrenceUtil.cs	100% <100%> (ø)

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[axunonb]

Thanks for fixing and including this important test case