archive metadata plugin sends empty collocation hints

[labkode]

Description

The archive metadata plugin sends empty collocation hints to FTS.

The following have been observed in ATLAS and CMS FTS databases for multiple transfers:

MySQL [newfts3atlas]> select archive_metadata, count(*) from t_file group by archive_metadata;
+---------------------------+----------+
| archive_metadata          | count(*) |
+---------------------------+----------+
| NULL                      |    15780 |
| {"collocation_hints": {}} | 14437895 |
+---------------------------+----------+
2 rows in set (29.215 sec)

MySQL [newfts3atlas]

I think it could be a side effect that when no policy is configured, the default is to send an empty dict?

If there are no collocation hints, the archive metadata should not be sent.

Steps to reproduce

Observed in production env

Rucio Version

No response

Additional Information

No response

[labkode]

Thanks @voetberg !