How to reproduce:
1. Create a new Unity project
2. Import the Addressables package, create Addressable settings and mark anything as Addressable
3. Search for the AssetReferenceDrawer.cs script and open it
4. Set some breakpoints at the beginning of SetObject() function and attach debugger to Unity
5. Create a new script and add a public UnityEngine.AddressableAssets.AssetReference variable
6. Attach this script to any GameObject
7. In the Inspector, click on the AssetReference field and select an addressable - code execution hits EditorUtility.SetDirty
8. In the Inspector, click on the AssetReference field and select 'None (AddressableAsset)'

Expected result: code execution eventually hits EditorUtility.SetDirty
Actual result: object is never set to dirty when setting the reference to null

Reproducible with: 2018.4.24f1, 2019.4.2f1, 2020.1.0b14, 2020.2.0a16 - Addressables 1.2.2, 1.11.2
Not reproducible with: 2018.4.24f1, 2019.4.2f1, 2020.1.0b14, 2020.2.0a16 - Addressables 1.1.10

Notes:
Before 1.1.10 and earlier SetObject() would call SetEditorAssetWithUndo() which would hit EditorUtility.SetDirty for null targets