Collections containing deleted content breaks when retrieving via API

[shrimpza]

Summary

When you have an existing collection containing a mod that's been deleted, that collection cannot be accessed via the API, instead it serves a 500 Server Error response.

Steps to reproduce

I've kept a broken collection here for reference: https://content.luanti.org/collections/shrimp/broken/

It loads via the UI, however accessing this via the API at https://content.luanti.org/api/collections/shrimp/broken/ returns a 500 Server Error.