Unnecessary use of a null check ('!') on enum values.

Latest version (6.10.0 on Dart 3.8.0) generates the following code for an enum field

```dart
Map<String, dynamic> _$XyzToJson(Xyz instance) =>
    <String, dynamic>{
      'state': _$AbcEnumMap[instance.state]!,
    };
```

The generated `_$AbcEnumMap` is a const map with all the enums listed, and the linter now complains that it is an unnecessary operation.