Soft Object Reference Pins can't be reset to None

Soft Object Reference pins on Blueprint nodes can't be reset to None once an object reference has been set. 

1. Create a Blueprint node with a SoftObjectReference<UObject> pin. Variable setter, make struct, function parameter, etc.
2. Select an object using the asset picker dropdown on the pin.
3. Select (None) at the top of the list
4. Note that the selection is not cleared