Malformed empty result for GROUP BY query on Dataset

An empty SPARQL query with GROUP BY returns a malformed result row:

~~~python
from rdflib import Dataset
print(Dataset().query("SELECT ?s (count(?o) as ?n) { ?s <some:predicate> ?o } GROUP BY ?s").bindings)
~~~

output looks like result was one row but variable bindings are missing:

~~~
[{}]   
~~~

expected output:

~~~
[]   
~~~