Bug 40119: (QA follow-up) Small additional comment

The reference to this bug report may be helpful to understand
later why we did so.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 01526ef545)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>

C4/AuthoritiesMarc.pm

@@ -1749,7 +1749,8 @@ sub merge {
                 if ( exists $controlled_ind->{sub2} ) {    # thesaurus info
                     if ( defined $controlled_ind->{sub2} ) {
 
-                        # Add or replace
+                        # Add or replace when not empty. If empty, an existing
+                        # $2 value is retained (see also bug 40119).
                         if ( $controlled_ind->{sub2} ne q{} ) {
                             $field_to->update( 2 => $controlled_ind->{sub2} );
                         }