### 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
rot_init = Math::fposmod(rot_init, (float)Math_TAU);
							if (rot_init < Math_PI) {
								rot_a = rot_a > rot_init + Math_PI ? rot_a - Math_TAU : rot_a;
								rot_b = rot_b > rot_init + Math_PI ? rot_b - Math_TAU : rot_b;
							} else {
								rot_a = rot_a < rot_init - Math_PI ? rot_a + Math_TAU : rot_a;
								rot_b = rot_b < rot_init - Math_PI ? rot_b + Math_TAU : rot_b;
							}
							t->value = Math::fposmod(rot_a + (rot_b - rot_init) * (float)blend, (float)Math_TAU);
						} else {
							value = Animation::cast_to_blendwise(value);
							if (t->init_value.is_array()) {
								t->element_size = MAX(t->element_size.operator int(), (value.operator Array()).size());
							} else if (t->init_value.is_string()) {
								real_t length = Animation::subtract_variant((real_t)(value.operator Array()).size(), (real_t)(t->init_value.operator String()).length());
								t->element_size = Animation::blend_variant(t->element_size, length, blend);
							}
							value = Animation::subtract_variant(value, Animation::cast_to_blendwise(t->init_value));
							t->value = Animation::blend_variant(t->value, value, blend);
						}
					} else {
						if (seeked) {
							int idx = a->track_find_key(i, time, is_external_seeking ? Animation::FIND_MODE_NEAREST : Animation::FIND_MODE_EXACT);
							if (idx < 0) {
								continue;
							}
							Variant value = a->track_get_key_value(i, idx);
							value = post_process_key_value(a, i, value, t->object);
							t->object->set_indexed(t->subpath, value);
						} else {