Avoid indexing instances without a base in scene cull phase

[clayjohn]

Fixes: #95466
Fixes: #92615
Fixes: #80749
Fixes: #69258
Fixes: #80504
Supersedes: #81069

At the core, this crash happens because an instance without a base is added to one of our BVHs and then we try to operate on that instance in the other BVH.

There are 2 main ways this crash occurs:

1. The instance is updated while the base is still NONE. Then the base is set to a valid base type. And finally the base is set to an empty RID before the instance is updated. This can happen when freeing an instance the same frame it was created (Godot crashes during remove BVH leaf #69258) or by a Node that sets its base to RID() during update (CSG or Sprite3D both do this). This crash occurs
2. The instance is updated while the base is still NONE, then the base is changed to a geometry type. This places the instance into the VOLUME BVH instead of the GEOMETRY BVH. And so the engine crashes when it tries to update the instance in the GEOMETRY BVH later.

In both cases, the problem comes from the fact that the instance is added to the VOLUME BVH. The logic is: if an object is a GEOMETRY type, it goes in the GEOMETRY BVH, otherwise it goes into the VOLUME BVH.

When we go to set the base of an instance, the logic is: if the previous base type is not NONE then remove the instance from the BVH and clear all data. If the base type is NONE we are assuming it was never added to a BVH. However, that assumption is false.

This PR fixes the bug by making that assumption true. It adds an early out to _update_instance to ensure that when the base type is NONE, we don't add the instance to a BVH. This change is low risk since the instance gets queued for update again once the base type is set. Without a base, the instance can't do anything. So its safe to skip the pairing/indexing step when the base has not been assigned.

The solution proposed by jsjtxietian in #81069 appears to work well for the Sprite3D case. It moves the instance from the VOLUME BVH to the GEOMETRY BVH if the base changes from NONE to GEOMETRY. However, it doesn't solve the other cases of the crash.

CC @jsjtxietian

[jsjtxietian]

LGTM. My pr is just a hack anyway.

[BastiaanOlij]

LGTM, make perfect sense, there is no point adding this to either BVH as there is no data so I wonder why we added it originally.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.3.1.