RigidBody falling slightly with Gravity Scale set to 0

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Linux Godot 3 master 5f805de

Steps to reproduce:

• Create RigidBody with CollisionShape
• Set Gravity Scale to 0
• Run and see RigidBody falling before stopping

Link to minimal example project:
physics.zip

[CowThing]

Only happens with Bullet physics.

[akien-mga]

cc @AndreaCatania

[AndreaCatania]

Fixed: #13046