Remove backfill errors

[Reettaphant]

This pull request fixes two things:

• If a collection has a backfill of type collection where the collectionId refers to a parent collection that does not exist, don't return an error but just return an empty list for the backfill instead
• If a backfill has a backfill type that is not recognised, don't throw an error but return an empty backfill.

[piuccio]

Nice, that's how I should have implemented in the first place.

The parent collection might disappear at any point, this shouldn't prevent other fronts from pressing.

Returning empty list when backfill is not recognized means facia tool can create new types without breaking frontend

@janua

[janua]

:+1: