'optional string' in proto3 messages will not yield nullable strings in C#

syntax = "proto3";
import "google/protobuf/wrappers.proto";

message Sample {
  // this will throw if you don't pass at least an empty string in generated C# code
  optional string not_nullable = 1;

  // this wrapper generates `string?` in C# (as one would expect)
  google.protobuf.StringValue nullable = 2;
}