(Optional) ForeignKey fields aren't properly translated from the models to the api

[codingpaula]

Steps to reproduce: set up a minimal decision tree and set the color value on a leaf to a color with id 1. Now GET that tree (using /api/tree/latest for example).

Actual result: color is null

Expected result: color is 1

(Optional) Foreign Key fields are behaving strangely in this django-ninja version. I don't know if this is related to me using Python 3.8 that doesn't support setting my colore value to int | None.