### Godot version

v4.2.rc.custom_build [d6a1db2b0]

### System information

Godot v4.2.rc (d6a1db2b0) - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3090 Ti (NVIDIA; 31.0.15.4592) - AMD Ryzen 9 7950X 16-Core Processor (32 Threads)

### Issue description

`AnimatedValuesBackup::get_data()` can crash as the internal data map can have nullptr values if any of the tracks were METHOD or ANIMATION types, as `AnimatedValuesBackup::get_cache_copy()` defines that these types of tracks do not get valid copies.

The resulting crash is therefore a null pointer exception.

### Steps to reproduce

1. Open a project with these type of animation tracks.
2. Save the project.

### Minimal reproduction project

https://github.com/godotengine/godot-demo-projects/tree/master/3d/platformer
playing_caches.erase(t);
									player2->stop();
									t->playing = false;
								}
							} else {
								player2->play(anim_name);
								t->playing = true;
								playing_caches.insert(t);
							}
						}
					}
				} break;
			}
		}
	}
}
void AnimationMixer::_blend_apply() {
	
	for (const KeyValue<NodePath, TrackCache *> &K : track_cache) {
		TrackCache *track = K.value;
		if (!deterministic && Math::is_zero_approx(track->total_weight)) {
			continue;
		}
		switch (track->type) {
			case Animation::TYPE_POSITION_3D: {
#ifndef _3D_DISABLED
				TrackCacheTransform *t = static_cast<TrackCacheTransform *>(track);
				if (t->root_motion) {
					root_motion_position = root_motion_cache.loc;
					root_motion_rotation = root_motion_cache.rot;