Support empty response from swh-graph
Workaround to support the empty response of swh-graph when a revision has no parent
In this case, swh-graph should reply a 201 no-content and this response should be correctly interpreted by the swh-graph lib, but it the meantime, it'll allow provenance to work with swh-graph
Related to #4314 (closed)
Migrated from D7972 (view on Phabricator)