Fix attribute generation for DeReKo

Change-Id: I22be35749b00d684f8df4f69bdf6cbd58250e507
diff --git a/lib/KorAP/XML/Annotation/DeReKo/Structure.pm b/lib/KorAP/XML/Annotation/DeReKo/Structure.pm
index 7bd12f2..769e629 100644
--- a/lib/KorAP/XML/Annotation/DeReKo/Structure.pm
+++ b/lib/KorAP/XML/Annotation/DeReKo/Structure.pm
@@ -101,7 +101,7 @@
           # Add attributes
           $mtt->add(
             term =>
-              '@:dereko/s:' . $_->{'-name'} . ':' . $_->{'#text'},
+              '@:dereko/s:' . $_->{'-name'} . ($_->{'#text'} ? ':' . $_->{'#text'} : ''),
             p_start => $p_start,
             pti     => 17,
             payload => '<s>' . $tui .