[Bug] The v2 client shows an unset vendor tare as 0 g #1222

Vendor.empty_spool_weight is optional in the API, but the v2 mapper converts null or a missing value to 0. The vendor inspector and embedded vendor section then display 0 g, presenting an unknown tare as a recorded zero. The inspector also has no clear action, so a saved vendor tare cannot be unset through that field. Reproduction: create or open a vendor whose empty_spool_weight is null, then view it in the v2 Library. The empty spool weight appears as 0 g. Expected: unset remains blank or shows an unset marker, while an explicitly stored 0 continues to display as 0 g; the inspector should allow clearing the value.