[graphs] Clean up lints for Dart 3.3 (#221)

Author: dcharkes

pkgs/graphs/analysis_options.yaml

@@ -13,8 +13,6 @@ linter:
     - avoid_classes_with_only_static_members
     - avoid_private_typedef_functions
     - avoid_redundant_argument_values
-    - avoid_returning_null
-    - avoid_returning_null_for_future
     - avoid_returning_this
     - avoid_unused_constructor_parameters
     - avoid_void_async