Opportunistically skip log in `update_claims_view_from_matched_txn`

On each block, for each `ChannelMonitor`, we log two status
statements in `OnChainTx::update_claims_view_from_matched_txn`.
This can add up to quite a bit, and is generally not very
interesting when we don't actually do anything if there's no claims
to bump.

Here we drop both logs if we have no claims to work with, but
retain it if we process any claims.