duration_test.cc: Add test for printing zero duration
duration_test.cc: Add test for printing zero duration

It's somewhat counter-intuitive, but Cassandra also formats zero-valued
duration values as an empty string.

scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla