Feature: Migrate source code to null safety.

[320Jackson]

Feature Description

TAT was stay in Dart 2.10 (Flutter 3.7.12) since 2023. To improve compilation efficiency, reduce null check and prepare to upgrade Flutter SDK into newer version, we should migrate the source code into sound null safety.

After Dart 2.12, the developers need to enable null safety.
Latest versions Dart language enforces sound null safety.

Use Case

1. Use '?' symbol to denote nullable type. if the value is null, return null.
2. Use '!' symbol to denote the value is NOT null right now.

Proposed Implementation

https://dart.dev/null-safety#enable-null-safety
https://dart.dev/null-safety/migration-guide#switch-to-the-dart-2-19-6-release