Aggregate with Group By

Hello,

What is the expected result set for the following query if we suppose that
the graph pattern in WHERE does not match any triple;

SELECT (sum(?c) as ?s)
WHERE{
   ?a ?b ?c .
   FILTER (str(?a) = "something_that_not_exists")
}
GROUP BY ?a

If I am not wrong, the same query without the GROUP BY, must return error
(null).

Regards,
Dimis