String? test(int? foo) {} should be error?

the code is ok, but foo must put a argument, String? also must return a explict value?
```dart
void main(List args) {
print(test(null));
}

String? test(int? foo) {
// this should explictly return null?
}
```

Start with the Dart snippet in main and the test function, then examine the language rules for nullable parameters and return values. Done means a resolved specification decision stating whether this code should be accepted and what explicit-return behavior is required.

Written by the indexing model from the issue text.