Null objects aren't cached

[alexandrevryghem]

Describe the bug
Objects aren't cached when they return null. This is visible for example on the search page, where thumbnails are requested again even tough they were embedded.

To Reproduce
Steps to reproduce the behavior:

1. Go to the search page and open the network tab (ensure there are items without thumbnails)
2. You will see that even tough the results contain thumbnail: null an additional request is sent to retrieve who returns a 204

Expected behavior
No additional thumbnail requests should be sent