Bug 9755: (follow-up) tweak test for tag visibility in merge hash

Bug 9755: (follow-up) tweak test for tag visibility in merge hash

Koha's MARC framework structure doesn't assign tags to tabs in
the cataloging editor; only subfields are assigned to tabs, and
the field as a whole follows the subfields.

Consequently, for the purpoase of calculating a merge hash, a field
should be added either if no MARC framework information is passed
(which signals that the caller wants to deal with the entire MARC
record) or if any of its subfields are assigned to visible tabs.

This patch has the effect of getting rid of the following spurious
warnings from the Apache log:

merge.pl: Argument "" isn't numeric in numeric ge (>=) at /home/gmc/koha/Koha/Util/MARC.pm line 78

Signed-off-by: Galen Charlton <gmc@esilibrary.com>