package/loader: Handle dangling branches when processing last snapshot
requested to merge anlambert/swh-loader-core:package-loader-handle-dangling-branch-in-last-snapshot into master
Previously any package loader that produced a snapshot for an origin with dangling branches in it fails to load that origin again as None check was missing when processing the last snapshot.
Fix #3449 (closed) and SWH-LOADER-CORE-16D.