code: func isOptionalAlias(t *types.Type) bool {
  	if t.Underlying == nil || (t.Underlying.Kind != types.Map && t.Underlying.Kind != types.Slice) {
  		return false
  	}
  	if extractBoolTagOrDie("protobuf.nullable", t.CommentLines) == false {
  		return false
  	}
  	return true
  }
docs: // isOptionalAlias should return true if the specified type has an underlying type
  // (is an alias) of a map or slice and has the comment tag protobuf.nullable=true,
  // indicating that the type should be nullable in protobuf.