Under some circumstances (loading autosaves), we end up reading from
files that were saved with the new mesh format (after T95965). When
that happens we should skip the conversion from the old format to
avoid data-loss. This will also give forward compatibility when we
stop saving in the old format completely in 4.0.

Here I mostly just check if the attributes in the new format already
exist. Along with checking for the null status of Mesh::mvert, that
should cover the majority of cases.

Fixes T103878