### 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_info.length = a->get_length();
					track_info.time = time;
					track_info.volume += blend;
					track_info.loop = a->get_loop_mode() != Animation::LOOP_NONE;
					track_info.backward = backward;
					track_info.use_blend = a->audio_track_is_use_blend(i);
					HashMap<int, PlayingAudioStreamInfo> &map = track_info.stream_info;
					
					if (p_update_only) {
						continue;
					}
					
					int idx = -1;
					if (seeked) {
						idx = a->track_find_key(i, time, is_external_seeking ? Animation::FIND_MODE_NEAREST : Animation::FIND_MODE_EXACT);
						
						if (map.has(idx)) {
							t->audio_stream_playback->stop_stream(map[idx].index);
							map.erase(idx);
						}
					} else {
						List<int> to_play;
						a->track_get_key_indices_in_range(i, time, delta, &to_play, looped_flag);
						if (to_play.size()) {
							idx = to_play.back()->get();
						}
					}
					if (idx < 0) {
						continue;
					}