`GetConventionalCommitsSinceLastGeneration` do not need to search for commits if `LastGeneratedCommit` is empty

From [this](https://github.com/googleapis/librarian/pull/1867#discussion_r2307716883) discussion, we think it's common that new library doesn't have a `LastGeneratedCommit`.

We should skip fetching the commits in this case.