Fix non-empty, empty RDF.Graph when adding empty RDF.Description

[pukkamustard]

This fixes a minor bug that allowed the creation of a weirdly shaped RDF.Graph by adding an empty RDF.Description:

iex(37)> RDF.Graph.new(RDF.Description.new(~I<http://example.com>)) |> RDF.Data.statements()
[nil]

Expected outcome would have been [].

This manifested itself during property-based testing (and thus in a very unnatural setting).

Pull request fixes the bug and adds a test that would catch this behavior.

Note: based on v0.7.1 as I am not able to compile hackney dependency version for 0.8.* on my machine and can not run tests. But looks like it rebases without conflict on master.

[marcelotto]

👍 Good catch. Thanks for the fix.

Regarding your problems with 0.8 and hackney: I've also noticed this problem on CI, but with OTP 20 only. So, if you're running on this version maybe you want to upgrade to a newer OTP version. Otherwise, maybe this might help: benoitc/hackney#632 (comment)

[pukkamustard]

Regarding your problems with 0.8 and hackney: I've also noticed this problem on CI, but with OTP 20 only. So, if you're running on this version maybe you want to upgrade to a newer OTP version. Otherwise, maybe this might help: benoitc/hackney#632 (comment)

Thanks! That made it work for me as well.