Don't include `iconOverlays` in rolesData if it is empty

Don't include `iconOverlays` in rolesData if it is empty

If this role isn't in the model yet, it would be `QVariant::Invalid`
which is obviously a distinct type from an empty string list.

This means `KFileItemModel::setData` treats them non-equal and
potentially does expensive operations on the model, which is
called every time a role is resolved in `KFileItemModelRolesUpdater`.

With this change, the number of pointless layout calculations is
significantly reduced.