set conversion to work with empty sequence as well.

modules/fontane/misc.xqm

@@ -19,9 +19,9 @@ function f-misc:get-title-by-baseUri($baseUri as xs:string) as xs:string? {
     $f-misc:metadata-collection//tgmd:textgridUri[starts-with(string(.), $baseUri)]/ancestor::tgmd:object//tgmd:title/string()
 };
 
-declare function f-misc:nb-param-to-title($node as node(), $model as map(*), $nb)
+declare function f-misc:nb-param-to-title($node as node(), $model as map(*), $nb as xs:string?)
 as xs:string? {
-  if($nb = "")
+  if(string($nb) = "")
   then ""
   else f-misc:get-title-by-baseUri( "textgrid:" || $nb )
 };