empty prefix is not in the prefixes(-Pairs) of rdf_read_turtle/3

[josd]

The following TriG file has an empty namespace prefix

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
@prefix : <http://example.org/ns#>.

:A rdfs:subClassOf :B.

and reading it via rdf_read_turtle(stream(In), Triples, [base_uri(Arg), format(trig), prefixes(Pfxs), on_error(error)]),
works fine but the Pfxs is [rdfs-'http://www.w3.org/2000/01/rdf-schema#'] whereas I would expect it to be
[rdfs-'http://www.w3.org/2000/01/rdf-schema#', ''-'http://example.org/ns#']