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

Absent of any other information, I'll assume the following XML:

    
        item 1
        blue
    
    
        item 2
        
    
    
        item 3
    
    ...

A sample use case would look like: