godot: NodePath cant be compared with null

Operating system or device, Godot version, GPU Model and driver (if graphics related): Linux, 3d06957f12ba5c04702f3db980af9c07142e7886

Issue description: It’s impossible to compare NodePath with null. It will give Invalids operands 'NodePath' and 'Nil' in operator '=='. error

Steps to reproduce:

1. Create new scene.
2. Add Node type to the scene root
3. Attach this script to the Node:

extends Node
var someNodePath = NodePath();
func _ready():
	if(someNodePath == null):  #Invalids operands 'NodePath' and 'Nil' (...)
		pass
	else:
		print("it's ok");

1. Run the scene

Update 1: From my bisection it seems that 137f8a5 is the first bad commit