added non-nullability for core

packages/stream_chat_flutter_core/pubspec.yaml

@@ -8,7 +8,7 @@ issue_tracker: https://github.com/GetStream/stream-chat-flutter/issues
 publish_to: none
 
 environment:
-  sdk: ">=2.7.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
   flutter: ">=1.17.0"
 
 dependencies:
@@ -17,14 +17,15 @@ dependencies:
   meta: ^1.2.4
   rxdart: ^0.26.0
   stream_chat: ^1.5.0
+  collection: ^1.15.0-nullsafety.4
 
 dependency_overrides:
   stream_chat:
     path: ../stream_chat
 
 dev_dependencies:
-  fake_async: ^1.1.0
+  fake_async: ^1.2.0
   flutter_test:
     sdk: flutter
-  mockito: ^4.1.3
+  mockito: ^5.0.3