osd: guard max_element end() deref in get_health_metrics

slow_op_pools can be empty (all poolids 0/out-of-range) while
slow_op_types is not, making max_element return end(); dereferencing
it is UB.

Fixes: https://tracker.ceph.com/issues/78184
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>