Reproduction steps:
1. Open attached project (Assert)
2. Open scene
3. Click on the Main Camera
- Notice that there is a script attached with empty fields for Game Objects (Check Not Null and Check Null)
4. Click play
- In console you get error message that Check Null is Not Null, while it is null because no Game Object is added to Check Null
- If you add a Cube to both Check Not Null and Check Null you will still get the same message, and the same happens if you add a Cube to only one of them

Check Not Null (Assert.IsNotNull) should trigger when the value is not null
Check Null (Assert.IsNull) should trigger when the value is null