Iterating with AStar's get_available_point_id method

[almighty-atlas]

Godot version:
3.2

OS/device including version:
Windows 10 Pro 1909

Issue description:
When you try to iterate with the get_available_point_id method of the AStar class you get an index order of [1, 0, 2, 3, 4 ...] since the method returns 1 if empty.

Steps to reproduce:

1. Make a Project with a Scene with a TileMap and create an 8x8 grid for example.
2. Attach following script to it:

func _ready() -> void:
    var astar = AStar2D.new()
    var cells = get_used_cells()
    for cell in cells:
        astar.add_point(astar.get_available_point_id(), cell)
    print(astar.get_points())

3. Output:
   [1, 0, 2, 3, 4, 5, ...]

Minimal reproduction project:
See 2.