How can I know if my chunk return an empty value

Dear all,
I am caling a chunk with parameter


And here is my Chink code


In some case the returned value is empty.

I am looking for a condition to display a value if the return is empty an another value if the return is not empty.
Then I install the package ifEval, but I can not find a sutatble solution

or


Do you have an idea?
Thank

I may have misunderstood, but have you considered an output modifier?

http://rtfm.modx.com/display/revolution20/Input+and+Output+Filters+(Output+Modifiers)