Any empty object of the json::json_list type has its internal
_set variable assigned to false which results in such objects
being skipped by the json::json_builder.
Hence, the json returned by the api GET//compaction_manager/compaction_history
does not contain the field `rows_merged` if a cell in the
system.compaction_history table is null or an empty list.
In such cases, executing the command `nodetool compactionhistory`
will result in a crash with the following error message:
`error running operation: rjson::error (JSON assert failed on condition 'false'`
The patch fixes it by checking if the json object contains the
`rows_merged` element before processing. If the element does
not exist, the nodetool will now produce an empty list.
Fixes https://github.com/scylladb/scylladb/issues/23540Closesscylladb/scylladb#23514
113647550f