token_metadata: Shortcut zero leaving nodes case in calculate_pending_ranges_for_leaving
token_metadata: Shortcut zero leaving nodes case in calculate_pending_ranges_for_leaving

If there are zero leaving nodes, no need to calculate anything. This
saves time for calculating pending ranges in large clusters
significantly to avoid unnecessary calculation.

Refs #10337

Closes #10822

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla