reset button next in Path2D "curve" property not working.

[shafnaz]

Godot version:
3.2
OS/device including version:
Windows 10
Issue description:
The reset button does not reset to an empty curve. (See .gif animation)

Steps to reproduce:

1. Create "Path2D" node.
2. Draw your curve.
3. Now press the reset button. Curve disappears, but when you now draw a new curve, the reset button is also gone.
4. If you select "New Curve2D" from the dropdown, and then reset, it will default to the previous curve you had drawn.
   Minimal reproduction project:

[simpuid]

Same kind of bug is present in spatial path.
It can be fixed by using an empty Ref<Curve2D> and Ref<Curve3D> for curve property of Path2D and Path respectively.

@shafnaz If the above fix is applied then you have to assign a new Curve2D to Path2D through drop down menu before editing it or after a revert.

@KoBeWi If that sounds good to you then I will issue a PR (already tested).

[KoBeWi]

@simpuid If you have a PR ready, just open it and someone will review whether the fix is good or not.