title: How to know variable has value or not in XSLT
body: <p>I am creating XSLT file.
  I have one variable which take value from XML file.But it may happen that there is no reference in xml for the value and at that time XSL variable will return False/None(don't know).I want keep condition like,If there is no value for the variable use the default one.
  How to do that ? </p>