### 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
track_value->is_continuous |= anim->value_track_get_update_mode(i) != Animation::UPDATE_DISCRETE;
				track_value->is_using_angle |= anim->track_get_interpolation_type(i) == Animation::INTERPOLATION_LINEAR_ANGLE || anim->track_get_interpolation_type(i) == Animation::INTERPOLATION_CUBIC_ANGLE;
				
				
				
				bool skip_update_mode_warning = false;
				if (track_value->is_continuous) {
					if (!Animation::is_variant_interpolatable(track_value->init_value)) {
						WARN_PRINT_ONCE_ED("AnimationMixer: '" + String(E) + "', Value Track: '" + String(path) + "' uses a non-numeric type as key value with UpdateMode.UPDATE_CONTINUOUS. This will not be blended correctly, so it is forced to UpdateMode.UPDATE_DISCRETE.");
						track_value->is_continuous = false;
						skip_update_mode_warning = true;
					}
					if (track_value->init_value.is_string()) {
						WARN_PRINT_ONCE_ED("AnimationMixer: '" + String(E) + "', Value Track: '" + String(path) + "' blends String types. This is an experimental algorithm.");
					}
				}
				if (!skip_update_mode_warning && was_continuous != track_value->is_continuous) {
					WARN_PRINT_ONCE_ED("AnimationMixer: '" + String(E) + "', Value Track: '" + String(path) + "' has different update modes between some animations which may be blended together. Blending prioritizes UpdateMode.UPDATE_CONTINUOUS, so the process treats UpdateMode.UPDATE_DISCRETE as UpdateMode.UPDATE_CONTINUOUS with InterpolationType.INTERPOLATION_NEAREST.");
				}
				if (was_using_angle != track_value->is_using_angle) {
					WARN_PRINT_ONCE_ED("AnimationMixer: '" + String(E) + "', Value Track: '" + String(path) + "' has different interpolation types for rotation between some animations which may be blended together. Blending prioritizes angle interpolation, so the blending result uses the shortest path referenced to the initial (RESET animation) value.");
				}
			}
			track->setup_pass = setup_pass;
		}
	}
	List<NodePath> to_delete;
	for (const KeyValue<NodePath, TrackCache *> &K : track_cache) {
		TrackCache *tc = track_cache[K.key];
		if (tc->setup_pass != setup_pass) {