Hive Generator strips nullability of generics

**Steps to Reproduce**
When running `flutter pub run build_runner build`

**Code sample**
```
@HiveType(typeId: 1)
class HiveLocation extends HiveObject {
@HiveField(0)
CustomObject? address;
}
```

Provide a few simple lines of code to show your problem.
Hive Generator generates:
```
@override
HiveLocation read(BinaryReader reader) {
final numOfFields = reader.readByte();
final fields = {
for (int i = 0; i < numOfFields; i++) reader.readByte(): reader.read(),
};
return HiveLocation()..address = fields[0] as CustomObject?;
}
```

Cannot assign `CustomObject?` to `CustomObject?`.

This is fixed by making a change to `class_builder.dart`:
```
String _displayString(DartType e) {
return '${e.getDisplayString(withNullability: true)}';
}
```

I believe that the current implementation of `_displayString` is a remnant of pre-nullsafety code.

**Version**
- Platform: Mac, but probably all
- Flutter version: 2.2.3
- Hive Generator version: 1.1.1