support null safety

[yanashenyang]

The current Dart SDK version is 3.3.0.

Because flutter_dojo depends on date_format ^1.0.1 which doesn't support null safety, version solving failed.

The lower bound of "sdk: '>=1.20.1 <2.0.0 or >=2.0.0 <3.0.0'" must be 2.12.0 or higher to enable null safety.
For details, see https://dart.dev/null-safety