### 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
if (t->mesh_3d) {
					t->mesh_3d->set_blend_shape_value(t->shape_index, t->value);
				}
#endif 
			} break;
			case Animation::TYPE_VALUE: {
				TrackCacheValue *t = static_cast<TrackCacheValue *>(track);
				if (!t->is_continuous) {
					break; 
				}
				
				if (t->value.is_array()) {
					int actual_blended_size = (int)Math::round(Math::abs(t->element_size.operator real_t()));
					if (actual_blended_size < (t->value.operator Array()).size()) {
						real_t abs_weight = Math::abs(track->total_weight);
						if (abs_weight >= 1.0) {
							(t->value.operator Array()).resize(actual_blended_size);
						} else if (t->init_value.is_string()) {
							(t->value.operator Array()).resize(Animation::interpolate_variant((t->init_value.operator String()).length(), actual_blended_size, abs_weight));
						}
					}
				}
				t->object->set_indexed(t->subpath, Animation::cast_from_blendwise(t->value, t->init_value.get_type()));
			} break;
			case Animation::TYPE_BEZIER: {
				TrackCacheBezier *t = static_cast<TrackCacheBezier *>(track);
				t->object->set_indexed(t->subpath, t->value);
			} break;
			case Animation::TYPE_AUDIO: {
				TrackCacheAudio *t = static_cast<TrackCacheAudio *>(track);