Bug 17380: Graceful resolution of missing reporting tag in merge

Bug 17380: Graceful resolution of missing reporting tag in merge

Altough this patch deals with a mostly hypothetical case and this report
makes it practically impossible anymore to merge with records in the
Default framework (having no reporting tag), we can make the code of
sub merge still a bit more robust here.

If you would merge biblio records from one authtype to another and the new
framework would not have a reporting tag, before this patch the result would
be data loss. Merge would handle this request as a delete. This patch makes
merge handle it differently: instead of clearing the biblio records, it
keeps $a and $9 in order to make a future corrective merge possible.

Note: The additional condition on line 1468 for $tags_using_authtype
makes sure that we do not select all fields when the authtype should
unexpectedly be empty string (Default). This prevents crashing on
a "Control fields do not have subfields" error.

Test plan:
[1] Run t/db_dependent/Authorities/Merge.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>