[Feature Request] Use Dart null safety completely

Protobuf compiler introduced Dart null safety, why `StringValue`, `DoubleValue`, `Int32Value`, etc, still exists? Why not using
`String?`, `double?`, `int?`, etc?

The issue concerns replacing wrapper types such as StringValue, DoubleValue, and Int32Value with nullable Dart types after null safety was introduced. No files, tests, or entry points are identified, so first map where the protobuf compiler and runtime define and consume these wrappers; done would require an agreed scope and passing coverage for generated and runtime behavior.

Written by the indexing model from the issue text.