If you declare a property with a null value, it will be silently converted to not null

To reproduce:
1. Download attached project "905457.zip" and open in Unity
2. Open "Scene" scene
3. Create new Game Object and add the NullProperty component
Note: In Inspector you will see that the property called "NULL_PROPERTY" has a not-null value and that the property called "NULL_STRING" is null
4. Open NullProperty sript and in 21 line write "string foo = null;" and save it
5. Go back to Unity
6. Wait until sript reloads

Expected result: In Inspector you will see:

"NULL_PROPERTY == not null!"
"HIDDEN_NULL_PROPERTY == not null!"
"NULL_STRING == NULL"
"HIDDEN_NULL_STRING == NULL"

Actual result: In Inspector you will see:

"NULL_PROPERTY == not null!"
"HIDDEN_NULL_PROPERTY == not null!"
"NULL_STRING == not null!"
"HIDDEN_NULL_STRING == not null!"

Reproduced on Unity 5.5.3p2, 5.6.0p3 and 2017.1.0b2