CollisionPolygon2D with polygon point count < 3 throw issue in debug console

[TotCac]

Godot version:
current master

OS/device including version:
Windows 10 64bits

Issue description:
Creating a CollisionPolygon2D with polygon points < 3 throw error (not catched by editor) in debug console when saving scene or loading one
ERROR: canvas_item_add_polygon: Condition ' pointcount < 3 ' is true.
At: servers/visual/visual_server_canvas.cpp:668

Steps to reproduce:
Place CollisionPolygon2D with no additionnal properties and save the scene

Minimal reproduction project:
n/a

[Zylann]

This is expected, the node cannot work if it doesn't have enough points to form the simplest 2D polygon possible (a triangle).
Did you expect the editor to prevent the user from setting less than 3 points up-front?

[TotCac]

I expect having only a visual warning in the editor as it was before 2d meshes commit, not an error of the engine.
Because having an empty CollisionPolygon2D that you populate at runtime is needed sometimes.

[vlad-ed-git]

was about to post this issue too... it would be better if a warning was thrown not an error!

[Calinou]

Closing in favor of #45724 as it's more up-to-date.