[CAL-925] fix: add null check for userSchedule.availability

[ShaneMaglangit]

What does this PR do?

Fixes #6708

This issue seems to appear when the user does not have any availability created.

• Does not happen when using any of the pre-created accounts, even if all availability are removed.

Environment: Production

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Chore (refactoring code, technical debt, workflow improvements)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How should this be tested?

• [ ] Test A
• [ ] Test B

Checklist

• I haven't commented my code, particularly in hard-to-understand areas
• I haven't added tests that prove my fix is effective or that my feature works

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Name	Status	Preview	Comments	Updated
ui	⬜️ Ignored (Inspect)			Feb 11, 2023 at 8:54AM (UTC)

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[emrysal]

Thank you for your contribution! I added a FIXME so we will come back to this later to handle the situation with no availability better; but this fixes the hard crash 👍