message: Update nullable attribute when upgrading link column
diff: @@ -239,6 +239,13 @@ bool Spec::convert_column_attributes()
  changes = true;
  break;
  }
  + case col_type_Link:
  + if (!attr.test(col_attr_Nullable)) {
  + attr.set(col_attr_Nullable);
  + m_attr.set(column_ndx, attr.m_value);
  + changes = true;
  + }
  + break;
  case col_type_LinkList:
  if (!attr.test(col_attr_List)) {
  attr.set(col_attr_List);
  