Cannot create collision mesh using accessors

[LeXXik]

The engine fails to create the collision mesh, when component's property accessors are used. This works:

this.entity.addComponent('collision', {
    type: 'mesh',
    renderAsset: this.app.root.findByName('cube').render.asset
});

This doesn't:

this.entity.addComponent('collision')
this.entity.collision.type = 'mesh';
this.entity.collision.renderAsset = this.app.root.findByName('cube').render.asset;

Repro:
https://playcanvas.com/project/1218921/overview/collision-mesh-issue

The issue is because base collision system implementation creates an empty model before initialization:

engine/src/framework/components/collision/system.js

Lines 49 to 54 in e0277bd

	beforeInitialize(component, data) {
	data.shape = null;
	data.model = new Model();
	data.model.graph = new GraphNode();
	}

Then when we try to assign a render asset, it first looks into model, which exists, but has no mesh instances and as a result fails to create a collision mesh:

engine/src/framework/components/collision/system.js

Lines 450 to 466 in e0277bd

	createPhysicalShape(entity, data) {
	if (typeof Ammo === 'undefined') return undefined;
	if (data.model || data.render) {
	const shape = new Ammo.btCompoundShape();
	const entityTransform = entity.getWorldTransform();
	const scale = entityTransform.getScale();
	if (data.model) {
	const meshInstances = data.model.meshInstances;
	for (let i = 0; i < meshInstances.length; i++) {
	this.createAmmoMesh(meshInstances[i].mesh, meshInstances[i].node, shape, null, data.checkVertexDuplicates);
	}
	const vec = new Ammo.btVector3(scale.x, scale.y, scale.z);
	shape.setLocalScaling(vec);
	Ammo.destroy(vec);