Feature request - support Optional data type

[JamesMcIntosh]

When creating the model it would be great to have nullable columns handled with the Optional so nullability becomes explicit and null checks can be removed from the code.

import 'package:optional/optional.dart';

class NullableClass extends Equatable {
  @Column(isNullable: true)
  Optional<String> nullableValue;

  NullableClass({this.nullableValue}) : super([nullableValue]);

}

Current behaviour is an Exception which is logged in the generate .jorm.dart files, see issue #127

[JamesMcIntosh]

Another area which Optional could be introduced is in the generated bean's find

Future<Optional<?>> find(int id)

Future<?> get(int id)

[tejainece]

Will look into this.

[jaumard]

dart will (soon?) have nullable field so it will be better to wait for it and use it once it's there instead of playing around and add an Optional class that will complexify the core code