[CHORE] Add null-safety to registries and 5 other classes funkin.data

[VioletSnowLeopard]

A contribution towards #4303.

This PR adds null-safety to all registries in funkin.data (SongRegistry already has null-safety). These include:

• BaseRegistry
• ConversationRegistry
• DialogueBoxRegistry
• SpeakerRegistry
• SongEventRegistry
• AlbumRegistry
• PlayerRegistry
• FreeplayStyleRegistry
• NoteStyleRegistry
• StageRegistry
• LevelRegistry
• StickerRegistry

I also had to fix Song.hx because that file has null-safety, but was dependent on BaseRegistry which wasn't null-safe.

Aside from the registries, I included null-safety for some other classes in funkin.data too. These include:

• DataError
• DataParse
• DataWrite
• AnimationData
• SongDataUtils

[Lasercar]

Just something to note, this'll have merge conflicts with 0.6.3, which to my understanding is being released... now