Version occasionally reverting to initial versions

[chingor13]

It appears that when we are using Manifest.fromConfig(), our search for the latest release version can return invalid results.

1. We grab the latest X commits on the target branch
2. We look for a merged release pull request
3. We fallback to latest GitHub releases that have a SHA matching a recent commit
4. We fallback to latest GitHub tags that have a SHA matching a recent commit

If step 1 fails (GraphQL), then we have an empty commit set, so step 2, 3, and 4 all fail. Thus release-please assumes we've never created releases before and propose a new release with the initial default version.

Example: googleapis/java-storage#1564 (look at the initial proposed PR).

[chingor13]

If step 1 fails, we probably want to error out in this case.

[suztomo]

This happened to googleapis/java-notification#643