migrate dart to null-safety

[marinkobabic]

NullSafety support for this library

Motivation:

In the newest versions null safety is a standard

Modifications:

• migrate all dart files
• update dependencies
• increase version to indicate breaking change

Result:

official nullsafety support of the library. You need to publish the new version

[marinkobabic]

@linux-china Please can you provide this version on pub.dev?

[linux-china]

@marinkobabic https://pub.dev/packages/rsocket published