Migrate to Null Safety

[felixgabler]

With the release of Flutter 2, it would be awesome if this library could be migrated to be null safe.

More details:
https://dart.dev/null-safety/migration-guide

[marchellodev]

Hey, I have made a fork based on this package, which has all the transition types that are here + a few more.

https://github.com/marchellodev/flutter_parallax_transitions

(it is migrated to null-safety and has a couple of other improvements)