Fixes a bug introduced in CL/432455486, which stopped saving explicit inertias if they were inferred from geoms. But if we don't save inertias then we need to save mass/density specifiers.
BEGIN_PUBLIC
xml_native_writer saves geom mass or density, if specified.
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PiperOrigin-RevId: 450667775
Change-Id: Ibb5b269b58f1143013091729642bb501243ed943