expression builder in BPEL

hello i am a newbie in oracle BPEL and also newbie in xpath, constantly i stuck at this if condition

if (strNOREF == null || (strNOREF != null && strNOREF.trim().length() == 0))
return Response.status(HttpStatus.FORBIDDEN.value()).entity("No. Ref is empty").build();

i wonder how do i translate into BPEL expression?

i tried this with this expression

$inputVariable.payload/ns2:strNOREF = "" or ($inputVariable.payload/ns2:strNOREF != "" and string-length($inputVariable.payload/ns2:strNOREF) = 0)

but it end up at else. i want to end up at my error message