Fix issue with Attendee not being string crashing tooltip

[masonbrothers]

There is an issue when you upload a new ics file to nextcloud calendar that does not contain attendees.

Since there is an assumption that the attendee is going to be a string (and not undefined), the tooltip crashes when you try to edit an event.

TypeError: Cannot read properties of undefined (reading 'startsWith')
    at b9 (attendee.js:30:10)
    at o.userAsAttendee (EditorMixin.js:234:22)
    at e.get (vue.runtime.esm.js:3446:33)
    at e.evaluate (vue.runtime.esm.js:3547:27)
    at o.userAsAttendee (vue.runtime.esm.js:5537:25)
    at o.isViewedByAttendee (EditorMixin.js:222:16)
    at e.get (vue.runtime.esm.js:3446:33)
    at e.evaluate (vue.runtime.esm.js:3547:27)
    at o.isViewedByAttendee (vue.runtime.esm.js:5537:25)
    at o.<anonymous> (EditSimple.vue:1:4915)

This PR should fix this by coercing non strings to become strings.

[codecov]

Codecov Report

Patch coverage: 9.89% and project coverage change: +0.03 🎉

Comparison is base (2687af3) 22.42% compared to head (12deb19) 22.46%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5170      +/-   ##
============================================
+ Coverage     22.42%   22.46%   +0.03%     
  Complexity      373      373              
============================================
  Files           237      237              
  Lines         11570    11575       +5     
  Branches       2174     2176       +2     
============================================
+ Hits           2595     2600       +5     
  Misses         8975     8975              

Flag	Coverage Δ
javascript	13.94% <9.89%> (+0.03%)	⬆️
php	63.94% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...nts/AppNavigation/CalendarList/CalendarListNew.vue	0.00% <0.00%> (ø)
...ponents/Subscription/HolidaySubscriptionPicker.vue	0.00% <0.00%> (ø)
src/store/calendars.js	0.00% <0.00%> (ø)
src/services/caldavService.js	31.74% <31.25%> (ø)
src/utils/attendee.js	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[miaulalala]

Thanks for the PR!

[miaulalala]

/backport to stable3.5

[pierreozoux]

Thanks a lot \o/ Have a wonderful weekend!