Origin visit API: raises an exception when a null branch pointer exists

https://archive.softwareheritage.org/api/1/origin/65629228/visit/145/ is a partial visit of the glibc Debian package (one version failed to import as it disappeared from the archive before we could download it).

This generated a branch pointing to a null object. When trying to access this object, the API fails.

This is possibly related to the changes in #949 (closed)

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Migrated from T972 (view on Phabricator)