[Bullet] None of the shapes in PhysicsServer (Bullet) are initializing their members

[Zylann]

Godot 3.1 ab843b1

Following #25301, I decided to create an issue for this too.
I've been noticing it for a while, but never actually had a problem with it until now:

In shape_bullet.cpp, none of the shapes initialize their members. That means if you create a shape with a script using PhysicsServer directly, and add it to a body without calling set_data, the implementation is potentially creating a Bullet shape with garbage values.
In particular, the HeightmapShape ends up with absurd sizes and a null pointer, which should have made Bullet assert but didnt because of #25301.

Is this API really requiring the user to call set_data before adding the shape because it has no defaults? I would assume no because Godot physics does initialize them.
Should we give defaults to all shapes at server level? (for heightmaps, a 1x1 square since Bullet forbids 0 and due to the server's implementation, the shape is forced to create a Bullet collider).