When no visit info (timestamp, id or snapshot) is provided to that function
return the last full visit with a valid snapshot or the last partial visit
with a valid snapshot otherwise.
This fixes a browsing issue when an origin is no more available but swh keeps
visiting it afterwards.
For instance the origin https://github.com/baifanwudi/PythonCode has been removed
since the first swh visit. Currently, when trying to browse it, we get
and empty snapshot view.
After that change, the last full visit with non empty snapshot will be displayed instead.