[stable4.7] 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 15.51%. Comparing base (2b1f393) to head (dc91540). Report is 3 commits behind head on stable4.7.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable4.7 #6486 +/- ## ========================================== Coverage 15.51% 15.51% ========================================== Files 206 206 Lines 9256 9256 Branches 1931 1931 ========================================== Hits 1436 1436 Misses 7820 7820 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/6486/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/6486/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.51% <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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