failure_detector_loop_for_node may be started on a shard before id->ip
mapping is available there. Currently the code treats missing mapping
as an internal error, but it uses its result for debug output only, so
lets relax the code to not assume the mapping is available.
Fixes#23407Closesscylladb/scylladb#24614
(cherry picked from commit a221b2bfde)
Closesscylladb/scylladb#24768