Make hash-checking failure mode stricter and safer (#2997)
## Summary
If there are no hashes for a given package, we now return
`Validate(&[])` so that the policy is impossible to satisfy. Previously,
we returned `None`, which is always satisfied.
We don't really ever expect to hit this, because we detect this case in
the resolver and raise a different error. But if we have a bug
somewhere, it's better to fail with an error than silently let the
package through.
