Check if a string is null or empty in XSLT

Question

How can I check if a value is null or empty with XSL?

For example, if categoryName is empty? I\'m using a when choosing construct.

For

Answer 1

First two deal with null value and second two deal with empty string.

    USERNAME is not null


    USERNAME is null
 
 
     USERNAME is empty string
 
 
     USERNAME is not empty string