fix: exception when saving user

[crnkovic]

Summary

https://app.clickup.com/t/862kh364f

If no permissions have been selected, the permissions key in the payload will be null. We used Arr::get() which will return the value if it exists, and since it does exist (but it's null), the exception happens.

I used null coalesce operator, which will default to an empty array if value doesn't exist or is null.

Checklist

• I checked my UI changes against the design and there are no notable differences
• I checked my UI changes for any responsiveness issues
• I checked my (code) changes for obvious issues, debug statements and commented code
• I opened a corresponding card on Clickup for any remaining TODOs in my code
• I added a short description on how to test this PR (if necessary)
• I added a storybook entry for the component that was added (if necessary)
• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged