When every resource dep of a node re-enters an ancestor's lock, `owned_reqs`
is empty; the old code still called `acquire(vec![])` and stored a no-op
empty guard in `owned`. Gate the acquire + guard insertion on
`!owned_reqs.is_empty()` — one fewer `borrow_mut` + `HashMap` entry per
fully-re-entrant node in the settle loop. `node_owns` already treats a
missing `owned` entry as non-owning, so behaviour is unchanged.
02167caf60