debugindexstats: handle the lack of Rust support better We don't have any stats in the Rust index. Currently it is not known which stats would be interesting to get, so if they end up being important, we can add them later.

rust-python-index: don't panic on a corrupted index when calling from Python This makes `test-verify.t` pass again. In an ideal world, we would find the exact commit where this test breaks and amend part of this change there, but this is a long enough series.