How to reproduce:
1. Open the attached project("AnimatorControllerBug")
2. Select GameObject in Hierarchy
3. Change the Animator component Controller value to None

Expected result: the Controller field becomes highlighted
Actual result: the Controller field doesn't change

Reproducible with: 2017.4.29f1, 2018.4.2f1, 2019.1.7f1, 2019.2.0b6, 2019.3.0a6

Notes:
- In debug inspector mode the Controller field is highlighted.
- Only the Controller field of Animation is affected. If a script containing Runtime Animator Controller field is created and the field is changed, the field is marked.
- In Overrides window, when Animator Controller field is changed it is shown that changes were made, but the changed Controller field is not highlighted.