### 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 (len > pasi.len) {
								stop = true;
							}
						}
						if (stop) {
							
							t->audio_stream_playback->stop_stream(pasi.index);
							erase_streams.push_back(M.key);
						} else {
							t->audio_stream_playback->set_stream_volume(pasi.index, db);
						}
					}
					for (uint32_t erase_idx = 0; erase_idx < erase_streams.size(); erase_idx++) {
						map.erase(erase_streams[erase_idx]);
					}
					if (map.size() == 0) {
						erase_maps.push_back(L.key);
					}
				}
				for (uint32_t erase_idx = 0; erase_idx < erase_maps.size(); erase_idx++) {
					t->playing_streams.erase(erase_maps[erase_idx]);
				}
			} break;
			default: {
			} 
		}
	}
}
void AnimationMixer::_call_object(Object *p_object, const StringName &p_method, const Vector<Variant> &p_params, bool p_deferred) {