Migrated to null safety

[FTKhanFT]

Upgraded the package to null safety.

[juliavi]

@benhurott
@calebeoliveira
@ElZombieIsra
Can you please review and merge the pull request? as Null-Safety is needed

[juliavi]

@benhurott
@calebeoliveira
@ElZombieIsra

[jsanz1209]

Thanks for this custom widget library, it's very nice.

Could you merge this pull request?

Thanks

[FTKhanFT]

If anyone still wants to use this package you can use it from git just add below snippet in your pubspec.yaml
Fully Nullsafety Supported

flutter_masked_text:
    git:
      url: https://github.com/FTKhanFT/flutter-masked-text
      ref: 0.8.2