Migrate to sound null safety

[limcheekin]

First check whether our app and the dependencies are ready to be migrated or not with the following command:

flutter pub outdated --mode=null-safety

If all dependencies are sound null safety, refer to the following article for next migration steps:
https://joshuawellbrock.medium.com/migrate-an-existing-flutter-app-to-flutter-2-0-74ce010eb013

[limcheekin]

Given the nature of many dependencies of the app, there's no way it can be migrated to sound null safety in one go, I think module by module is more practical as mentioned in Flutter Multi-modules Support blog post.

I will only start look into null safety migration after the modules migration completed.

[limcheekin]

completed in 0.6.0 released at 7a98975