Bug 36976: Warning 'Argument "" isn't numeric in numeric' in log when merging bibliographic records

When merging bibliographic records, Koha generates warning:

[WARN] Argument "" isn't numeric in numeric ge (>=) at /kohadevbox/koha/Koha/Util/MARC.pm line 81.

in [plack-]intranet-error.log.

This is because $tagslib->{$fieldtag}->{'tab'} -- for a tag, not for a
subfield --is (always?) an empty string (cf.
C4::Biblio::GetMarcStructure : $res->{$tag}->{tab} = ""), and in
Koha::Util::MARC::createMergeHash it is compared numerically with 0.

Test plan:
==========
1. From biblio search result page, selact two records with the checkboxes
   and start a merge (Edit -> Merge records).  Proceed with merging.
   After the merge, controll the content of the plack-intranet-error.log
   (when using plack -- standard in ktd).  Note the lines:
   [WARN] Argument "" isn't numeric in numeric ge (>=) at /kohadevbox/koha/Koha/Util/MARC.pm line 81.
2. Apply the patch; restart_all.
3. Repeat p. 1.  There should be no fresh warnings.

Sponsored-by: Ignatianum University in Cracow
Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit fb2e54b0c3)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

Koha/Util/MARC.pm

@@ -82,7 +82,7 @@ sub createMergeHash {
                 (
                     !defined($tagslib) || ( defined $tagslib->{$fieldtag}
                         && defined $tagslib->{$fieldtag}->{'tab'}
-                        && $tagslib->{$fieldtag}->{'tab'} >= 0 )
+                        && ( $tagslib->{$fieldtag}->{'tab'} ? $tagslib->{$fieldtag}->{'tab'} : 0 ) >= 0 )
                 )
                 && @subfield_array
               )