code: func NewNullFieldDescriptor(nullable NullableColumn) FieldDescriptor {
  	// A null field can be nullable ...
  	return newFixedLengthFieldDescriptor(
  		mysql_proto.FieldType_NULL,
  		nullable,
  		0,
  		func(b []byte) interface{} { return nil })
  }
docs: // This returns a field descriptor for FieldType_NULL (i.e., Field_null)