[stable 4.7] fix: check if userId is null

[hamza221]

manual backport of #6115

[codecov[bot]]

Codecov Report

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

Project coverage is 15.49%. Comparing base (9291d4a) to head (022b6be). Report is 3 commits behind head on stable4.7.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable4.7 #6119 +/- ## ========================================== Coverage 15.49% 15.49% ========================================== Files 206 206 Lines 9256 9256 Branches 1930 1930 ========================================== Hits 1434 1434 Misses 7822 7822 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/6119/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/6119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.49% <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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