Feature/null safety

[HaoCherHong]

migrate to sound null safety upgrade dependencies to sound null safety:

• xml: ^6.0.1
• cryptoutils: ^0.5.0
• crypto: ^3.0.2
• synchronized: ^3.0.0+3
• tuple: ^2.0.1
• unorm_dart: ^0.2.0
• image: ^3.2.2
• quiver: ^3.1.0 #hash code generator
• console: ^4.1.0 #for example
• intl: ^0.18.0
• collection: ^1.15.0-nullsafety.4

Also fix https://github.com/vukoye/xmpp_dart/pull/83

separate message time with delay timestamp

[TatankaConCube]

we have the branch null_safety and it contains all the latest changes, you can target your PR on it

[HaoCherHong]

we have the branch null_safety and it contains all the latest changes, you can target your PR on it

What's the progress of the branch? Any plan to release it?

[TatankaConCube]

it already released as 0.4.4-dev.2