empty braces in ConstructTemplate

Hi all,
I'm studing SPARQL 1.1 Grammar, and I notice that according with rule 
[73] I can write a Construct query like this

CONSTRUCT { }

I would like to know what is the semantic in that situation, when there 
are empty braces. I don't find it in the document.

Thank you
-Serena-

ps. The same rule is present in 1.0 version of SPARQL.