use_late_for_private_fields_and_variables should have tests with patterns

[srawlins]

Child issue of #58837

I think there should be a test case with an assignment pattern like:

int? _i;
m() {
  _i?.abs();
  (_i, ) = (null, );
}

in which the lint rule would allow that _i can be nullable, as it is assigned null, in a pattern.