message: Preserve subcolumn attributes (i.e. nullability) when migrating to lists
diff: @@ -233,7 +233,7 @@ bool Spec::convert_column_attributes()
  ref_type ref = to_ref(m_oldsubspecs.get(subspec_ndx)); // Throws
  sub_spec.init(ref);
  m_types.set(column_ndx, sub_spec.get_column_type(0));
  - m_attr.set(column_ndx, m_attr.get(column_ndx) | col_attr_List);
  + m_attr.set(column_ndx, m_attr.get(column_ndx) | sub_spec.m_attr.get(0) | col_attr_List);
  sub_spec.destroy();
  m_oldsubspecs.erase(subspec_ndx);
  changes = true;
  