[stable5.0] fix: handle timezones with no transitions properly

[backportbot[bot]]

Backport of PR #6478

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 15.43%. Comparing base (27997b8) to head (e352ef0). Report is 3 commits behind head on stable5.0.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable5.0 #6487 +/- ## ========================================== Coverage 15.43% 15.43% ========================================== Files 206 206 Lines 9441 9441 Branches 2204 2204 ========================================== Hits 1457 1457 Misses 7665 7665 Partials 319 319 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/6487/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/6487/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.43% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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