If start_time/end_time is unspecified for a task, task_manager API
returns epoch. Nodetool prints the value in task status.
Fix nodetool tasks commands to print empty string for start_time/end_time
if it isn't specified.
Modify nodetool tasks status docs to show empty end_time.
Fixes: #22373.
Closesscylladb/scylladb#22370