Noone really knows what the exact Chinese policy is there they like their strategic ambiguity but one thing’s for sure they are really big on non-proliferation, and thus aren’t exactly fans of nuclear blackmail.
I don’t think the Chinese would be triggered by Russia nuking its own territory, but then, well, Russia would be nuking its own territory. They could nuke Sudja to get rid of the incursion but they’d be nuking their own defence. Also, their own city. If they withdraw their defence first, Ukraine would gain even more territory and they’d have to nuke even more. Or, differently put: Just because the term “tactical nuke” exists doesn’t mean that nukes are sensible tactical weapons.