Fix: iCal integration fails with all-day events

[sasagar]

Proposed change

This PR fixes a issue of iCal integration.

For the one-day event with Google Calendar, the dtend would not defined. So the error appeared.
This PR fixes about it: when dtend is not defined, set it as same as dtstart.

And also, if dtend and dtstart are the same, set duration to 1;

Closes #2882

Type of change

• New service widget
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation only
• Other (please explain)

Checklist:

• If applicable, I have added corresponding documentation changes.
• If applicable, I have reviewed the feature and / or service widget guidelines.
• I have checked that all code style checks pass using pre-commit hooks and linting checks.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.

[shamoon]

Thanks, I already had a fix for this as someone reported it earlier, so I've just merged that in here instead of closing this PR. I think it's simpler. Please let me know your thoughts

[shamoon]

Ok, well let me know if you have any concerns. Thanks again.

[github-actions]

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