When you create a database poller in the OSB you will encounter that empty values in the database will appear in your request as xsi:nil="true".

When calling another service with values like

<sch:voorvoegsel xsi:nil="true"/>

you will run into validation problems so you want to remove those elements. You can use XQuery to filter them out but when I tried to use the fn:nilled function, it didn't do what I expected it to do. When I looked around I found this blog: http://www.oradevblog.nl/?p=229 which explains why it doesn't work. Solution......you can define a is_nilled function of your own which does the trick. It looks like this:

declare function xf:is_nilled
  ( $element as element() )  as xs:boolean {
 
   if (fn:exists($element/@xsi:nil)) then
      fn:exists($element[@xsi:nil=fn:true()])
   else
      fn:false()
 };