updated empty string to undef to store empty description as NULL rather than empty string

misc-scripts/xref_mapping/XrefParser/RefSeqCoordinateParser.pm

@@ -380,7 +380,7 @@ sub run_script {
           my $xref_id = $self->add_xref({ acc => $acc,
                                           version => $version,
                                           label => $id,
-                                          desc => '',
+                                          desc => undef,
                                           source_id => $source_id,
                                           species_id => $species_id,
                                           dbi => $dbi,
@@ -411,7 +411,7 @@ sub run_script {
               my $tl_xref_id = $self->add_xref({ acc => $acc,
                                               version => $version,
                                               label => $acc,
-                                              desc => '',
+                                              desc => undef,
                                               source_id => $source_id,
                                               species_id => $species_id,
                                               dbi => $dbi,