fix: Make hasGCalError optional

[Dschoordsch]

Fixes: #8976

During testing I ran into "Cannot return null for non-nullable field hasGCalError" issues. This is not necessary because it's not a criitical field. Avoid these kind of mistakes by just making it optional.

Testing scenarios

• restart a team prompt via processRecurrence

Final checklist

• I checked the code review guidelines
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• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog