Fix GH-14343: Memory leak in xml and dom (#14347)

If there is no root, the namespace cannot be attached to it,
so we have to attach it to the old list.

This isn't a problem in "

Fix GH-14343: Memory leak in xml and dom (#14347)

If there is no root, the namespace cannot be attached to it,
so we have to attach it to the old list.

This isn't a problem in "new DOM" because namespaces are managed in a
separate structure there.

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