`unnecessary_parenthesis` false positive with null-aware `toString`

[oprypin]

• This fix covers most cases: Fix unnecessary_parenthesis with null aware expressions. dart-archive/linter#3902 by @asashour

But seems that this case still slips through:

• String foo() {
    int? a;
    return (a?.abs().sign).toString(); // Unnecessary use of parentheses.
  }

• String foo() {
    int? a;
    return a?.abs().sign.toString(); // 'String?' can't be returned from a function with return type 'String'
  }

Probably the complicating factor is that toString is available on Object? and the null-aware expression has another dot after it