[Sequencer] Do not build empty blocks

[frisitano]

Overview

We should implement logic that skips the production of empty blocks. This should be done to avoid the unnecessary overhead of proving and DA of empty blocks.

Potential soltuions:

1. Build blocks as is normally, done and then skip if the block has no transactions
2. Have handles to L1 messages and transaction pool, and do not build a block if there are no transactions.

The consensus is that option 1 is the preferred and simpler solution.

[jonastheis]

We should also introduce a flag that explicitly allows empty blocks.