[C#] Can´t change a C# Node variable to null in GDscript

[cookie647]

Godot version

v3.5.stable.mono

System information

Arch Linux

Issue description

You can't assign null from GDScript to a variable with the type Node in a C# script.

For example you have this code in C#:

using Godot;

public class MyNode : Node2D
{
	public Node my_var = new Node();
}

And you are trying to change this value from GDScript to null:

func _ready():
	$MyNode.my_var = null

The value of my_var would not be set to null but remain. This procedure worked in previous versions of Godot.

Steps to reproduce

1. Create a Node with code in C#.
2. Create a variable in that C# script with the type Node. Assign any Node to this variable.
3. Create a second Node with code in GDScript.
4. Try to assign null to the variable of the first Node from the second Node.
5. Check if the variable has been set to null. You will see that it hasn't.

Minimal reproduction project

ChangingToNull.zip

[raulsntos]

After bisecting, this seems to be a regression caused by b775573 (#53942).

I couldn't reproduce the issue in master.

[API-Beast]

Issue still persists in 3.5.2 and it affects the inspector as well. Resource [Export]s can't be set to null, pressing the "clear" context menu entry does nothing, same with the little "Reset" button next to the property.

Only workaround is using a text editor to manually set it to null. (And then reloading the project.)