[CHORE] Add null-safety to 5 classes in funkin.play

[VioletSnowLeopard]

A contribution towards #4303.

This PR adds null-safety to:

• Countdown
• PauseSubState
• GitarooPause
• ResultScore
• ScoreNum

Because GameoverSubState, ResultState, and PlayStatePlaylist are already null-safe, this PR makes it so almost all base funkin.play classes have null-safety. The last one would be PlayState. (That file is over 3000 lines long, so it would probably need its own dedicated PR)

(Even though PlayState isn't null-safe yet, I still added checks for its values in Countdown and PauseSubState)

Some Notes for ResultScore and ScoreNum

In ResultScore, there was a variable initialized without a value, but it was unused except in commented-out code, so I removed it and the commented-out code associated with it.

In ScoreNum, there were two public variables initialized without a value (a FlxTimer and a FlxTween), but they were only ever referred to once in the class and no other classes accessed these variables. This meant I could safely remove these variables and set the FlxTimer and FlxTween by themselves instead.

[AbnormalPoof]

friday night funkin null safety (all bosses)

[NotHyper-474]

I actually have a branch where I'm implementing null-safety for PlayState and LoadingState, it's nearly complete, just needs some final polishing.

I also wanted to include it in #4787 but felt it was too unrelated.