Fix acceptance tests (#106)

* test: allow no metadata in packages

Signed-off-by: Alfredo Deza <adeza@anchore.com>

* test: update centos json fixture

Signed-off-by: Alfredo Deza <adeza@anchore.com>

Co-authored-by: Alex Goodman <wagoodman@users.noreply.github.com>

test/acceptance/compare.py

@@ -29,7 +29,7 @@ class syft:
             packages.add(package)
             metadata[package.type][package] = Metadata(
                 # note: the metadata entry is optional
-                metadata=repr(entry.get("metadata")), sources=repr(entry["sources"])
+                metadata=repr(entry.get("metadata", "")), sources=repr(entry["sources"])
             )
         return packages, metadata