Only increase indices if we actually find notes

[nventuro]

In #9794 we're blindly incrementing recipient tag indices as soon as we see any logs with the corresponding tag. This means a third party could spam us with bad logs with the expected tags and cause for us to incorrectly increase the tag index. Notably, said third party would need to know the tagging secret, and therefore the secret encryption key (h + ivsk) of either sender or recipient, making this attack have low likelihood.

To prevent this, we should only increase the index if we actually find a note in the log. We may never get to do this if we instead choose to e.g. implement the ideas described in https://forum.aztec.network/t/on-note-discovery-and-index-coordination/7165.