feat!: Migrate firebase_ml_vision to sound null safety

[AlexHartford]

Description

• Updated package to sound null-safety.
• Updated example to sound null-safety.
• Addressed various TODOs throughout the project.

I only have an Android emulator available to me for the next couple of weeks so it'd be good if someone could test the example project on iOS.

Related Issues

#4182

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Breaking Change

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[Salakar]

@AlexHartford would you mind resolving the conflicts, sorry. I took a look at doing it myself but wasn't too familiar with the changes done so didn't want to break your PR 🙃

[AlexHartford]

Handled the merge conflicts. Let me know if there's anything else you need! 😁

[rrousselGit]

Sorry for the delay, LGTM!

[AlexHartford]

Any plans to complete the merge soon?

[Salakar]

This has now been published to pub.dev - thanks everyone.