dird: cats: abort purge when there are no eligible jobids

[alaaeddineelamri]

Description

When pruning/purging jobs or files, Bareos does not check whether the list of jobids it provides to the SQL queries is empty or not, which makes the queries fail and Postgres emitting errors in the logs

This PR adds checks for empty jobids lists when purging and pruning.

Fix issue #1516

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[bruno-at-bareos]

Thanks for the work, now time to prepare the backports.

[bruno-at-bareos]

Approved.