extend INVALID_NULL_AWARE_OPERATOR for extensions on nullable

[a14n]

Following up on dart-lang/linter#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
}

[a14n]

https://dart-review.googlesource.com/c/sdk/+/243939

[albertms10]

The code block in the description contains syntax issues. One way to go may be:

extension on String? {
  bool get isNullOrEmpty => this == null || this!.isEmpty;
}

[a14n]

@albertms10 Thanks for letting me know. I updated the snippet.

[a14n]

Closing to migrate to linter.