Don't copy AuxDyn when container thinned down to 0. related to ATLASRECTS-7656
That change prevents the creation of empty AuxDyn branches when writing (reduces file size)
and makes xAOD copying (e.g. when merging files) a 1 to 1 operation (merging was dropping these empty branches)
see ATLASRECTS-7656
This change is fully transparent and does not affect data, but the file structure will be different (no more branches for not existing dynamic attributes).