Avoid returning null for voidDRT-W1066

AVOID returning null for void.

In a large variety of languages void as return type is used to indicate that a function doesn't return anything. Dart allows returning null in functions with void return type but it also allow using return; without specifying any value. To have a consistent way you should not return null and only use an empty return.

BAD:

void f1() {
  return null;
}
Future<void> f2() async {
  return null;
}

GOOD:

void f1() {
  return;
}
Future<void> f2() async {
  return;
}