Use null instead of an object with undefined properties

[Elettrotecnica]

This will be coalesced to an empty object, which keeps the expected behavior. The previous default would trigger a warning in the console about it not being conform to the vec3 property type definition.

Warning is:

core:schema:warn Default value [object Object] does not match type vec3 in component ammo-body

[diarmidmackenzie]

Thanks for this.