[CASSANDRA-21538] Fix NullPointerException in nodetool listpendinghints

[Yukei7]

nodetool listpendinghints crashes with a NullPointerException when a hint is still pending for a host id that is no longer part of the ring (the node was removed, decommissioned or replaced).

Fix: guard the null address, skip the snitch/gossip lookups in that case, and report the location fields (Address/Rack/DC/Status) as "Unknown" while still listing the pending hint.

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