### 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
float value;
					Error err = a->try_blend_shape_track_interpolate(i, time, &value);
					
					if (err != OK) {
						continue;
					}
					value = post_process_key_value(a, i, value, t->object, t->shape_index);
					t->value += (value - t->init_value) * blend;
#endif 
				} break;
				case Animation::TYPE_VALUE: {
					if (Math::is_zero_approx(blend)) {
						continue; 
					}
					TrackCacheValue *t = static_cast<TrackCacheValue *>(track);
					if (t->is_continuous) {
						Variant value = a->value_track_interpolate(i, time);
						value = post_process_key_value(a, i, value, t->object);
						if (value == Variant()) {
							continue;
						}
						
						if (t->is_using_angle) {
							
							
							float rot_a = t->value;
							float rot_b = value;
							float rot_init = t->init_value;
							rot_a = Math::fposmod(rot_a, (float)Math_TAU);
							rot_b = Math::fposmod(rot_b, (float)Math_TAU);