Empty prefix not returned by parser

[wouterbeek]

The Turtle/TriG parser does not return the empty prefix:

prefix : <https://example.org/>
prefix a: <https://example.org/a#>

?- use_module(library(semweb/rdf_db)).
?- use_module(library(semweb/turtle)).
?- rdf_load('test.trig', [prefixes(L)]),

Observe that L only contains the non-empty prefix declaration.