`use_null_aware_elements` could trigger for nullable spread cases too

Repro:

```dart
void f(List<Object>? nL, List<Object> l, int? i) {
  var _ = <Object>[
    ...l,
    ...?nL,
    if (nL != null) ...nL, // nothing but could be as above
    if (i != null) i, // use_null_aware_elements
    ?i,
  ];
}
```

CC @bwilkerson @kallentu 