Bug 7722 - Insidious problem with searching

I cannot find the root cause of this issue, but multiple libraries that I am aware of have problems searching on particular search terms ( and never the same terms at the same library ). The error they get when they trigger this problem is:

Tag "" is not a valid tag. at /home/koha/kohaclone/C4/Biblio.pm line 1849

Something somewhere is adding empty keys to C4::Context->marcfromkohafield, I think it may have something to do with the analytics feature that was added.

In the while loop for TransformKohaToMarc, there is a line

next unless my $dtm = $db_to_marc->{''}->{$name};

I don't think it's working.
If I dump $dtm, for each search, I see the dump twice.
It looks like this:
$VAR1 = [
           '952',
           'w'
         ];
 $VAR1 = [];
I think the second time, when it is empty is what's breaking this.
The next never fails because even though it is empty, it is still a valid arrayref.

The solution I have some up with is to skip over the elements where the arrayref is empty.

Signed-off-by: Ian Walls <koha.sekjal@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>

C4/Biblio.pm

@@ -1996,6 +1996,7 @@ sub TransformKohaToMarc {
     my $db_to_marc = C4::Context->marcfromkohafield;
     while ( my ($name, $value) = each %$hash ) {
         next unless my $dtm = $db_to_marc->{''}->{$name};
+        next unless ( scalar( @$dtm ) );
         my ($tag, $letter) = @$dtm;
         foreach my $value ( split(/\s?\|\s?/, $value, -1) ) {
             if ( my $field = $record->field($tag) ) {