Null to be Printed in between XML Tags

Hi,

I converting Intenal Table data into XML file.When we dont have any value for field,Null to be printed.

I am getting below format.

ex: value    ----> <vendor>1234<vendor/>

     No value ---><vendor/>

if vendor is empty then i need to print like as below

<vendor>NULL<vendor/>

how can i do like this????