Fix error when querying custom attributes on non-existing attributable

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What does this MR do and why?

Querying custom attributes on project, group or user caused a 500.

This MR fixes this behaviour and returns a 404 instead.

🛠️ with ❤️ at Siemens

References

• 500 response on GET custom attributes if projec... (#346403 - closed)

How to set up and validate locally

Before applying the MR the following requests should cause a 500, afterwards a 404. The suggested URL assumes there is no user with the specified id. The same applies to projects and groups.

1. List all custom attributes:

curl --request GET \
  --url https://gdk.test:3443/api/v4/users/10000/custom_attributes \
  --header 'Authorization: Bearer <glpat-...>'

2. List specific existing custom attribute:

curl --request GET \
  --url https://gdk.test:3443/api/v4/users/10000/custom_attributes/my-custom-location \
  --header 'Authorization: Bearer <glpat-...>'

3. Create/Update a specific custom attribute:

curl --request PUT \
  --url https://gdk.test:3443/api/v4/users/100000/custom_attributes/my-custom-location \
  --header 'Authorization: Bearer <glpat-....>' \
  --header 'Content-Type: application/json' \
  --data '{"value": "example"}'

MR acceptance checklist

checklist

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides