fix: hide "create task" option when no task calendar is available

[madbob]

The "Create task" modal ask for the calendar to which append the new task, but if none is available it is not possible to select anything and the whole operation cannot be concluded.

Fixes #9884

[ChristophWurst]

Thanks!

When are the XHRs for the collection listing happening and does this cause them to happen earlier?
In other words: any (performance) downsides?

[madbob]

User's calendars are loaded from the server as soon as the main Vue App is mounted, so there is not any extra DAV call.

[ChristophWurst]

Let's do this

[ChristophWurst]

/backport to stable5.10

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