Specify that focus neighbor "not set" means an empty NodePath, not null

**Your Godot version:**
4.1.1

**Issue description:**
I tried to set focus neighbor property on a Control node to null but it gave me an invalid set index error. This is because I need to have an empty nodepath (^"") not null.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_control.html#class-control-property-focus-neighbor-bottom