### 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
switch (track_type) {
									case Animation::TYPE_POSITION_3D: {
										track_xform->init_loc = reset_anim->track_get_key_value(rt, 0);
									} break;
									case Animation::TYPE_ROTATION_3D: {
										track_xform->init_rot = reset_anim->track_get_key_value(rt, 0);
									} break;
									case Animation::TYPE_SCALE_3D: {
										track_xform->init_scale = reset_anim->track_get_key_value(rt, 0);
									} break;
									default: {
									}
								}
							}
						}
#endif 
					} break;
					case Animation::TYPE_BLEND_SHAPE: {
#ifndef _3D_DISABLED
						if (path.get_subname_count() != 1) {
							ERR_PRINT("AnimationMixer: '" + String(E) + "', blend shape track does not contain a blend shape subname:  '" + String(path) + "'.");
							continue;
						}
						MeshInstance3D *mesh_3d = Object::cast_to<MeshInstance3D>(child);
						if (!mesh_3d) {
							ERR_PRINT("AnimationMixer: '" + String(E) + "', blend shape track does not point to MeshInstance3D:  '" + String(path) + "'.");
							continue;
						}
						StringName blend_shape_name = path.get_subname(0);
						int blend_shape_idx = mesh_3d->find_blend_shape_by_name(blend_shape_name);