Course list broken when course image in invalid state

[kesselb]

Steps to reproduce

1. Upload a course image
2. Delete the image object from file table (e.g. mongocli)
3. It's not possible to render list view for student/teacher/admin

TypeError: Cannot read property 'toObject' of null
    at model.<anonymous> (/home/daniel/Code/geli/api/build/src/models/Course.js:284:82)
    at Generator.next (<anonymous>)
    at fulfilled (/home/daniel/Code/geli/api/build/src/models/Course.js:4:58)

geli/api/src/models/Course.ts

Line 319 in c016a39

const image = (this.image) ? (await Picture.findById(this.image)).toObject() : null;

I'm not sure if its possible to force this error somehow (i did but can't remeber how). Code above checks if course.image is not null but not if there is a matching object in file table (not null) before calling toObject.

Acceptance criteria:

• No unsafe access