Increase privacy of off-chain notes with a minimum note per block requirement

[hackaugusto]

One of the downsides of the current sync endpoint is that privacy is only preserved if there is enough notes produced/consumed concurrently to prevent linkability.

One easy way of improving this for a new network is to allow the rollup sequencer to produce notes and include them into a block. For example, we currently seal a batches on regular intervals. And add notes from the pool to the batch. If there are no txs, an empty block is built. Instead, it could be done such that empty notes are generated, and added to the batches/blocks