add unnecessary_null_aware_operator_on_extension_on_nullable

[a14n]

Following up on #3379 (comment) to add a warning when using null aware operator with a member defined in an extension on nullable type.

extension on String? {
  bool get isNullOrEmpty {
    var self = this;
    return self == null || self.isEmpty;
  }
}

main() {
  String? s = null;
  s?.isNullOrEmpty; // WARN here
  s.isNullOrEmpty; // OK
}

Migrated from dart-lang/sdk#48980 into linter

[coveralls]

Coverage increased (+0.02%) to 95.656% when pulling 9ddb2d9 on a14n:unnecessary_null_aware_operator_on_extension_on_nullable into a3a265a on dart-lang:master.

[pq]

💎