[Bug] block manager creates empty blocks if there is no batch #2097

Issue Description

When using a sequencer, the block manager creates empty blocks locally when no batch is available. This behavior introduces inconsistency between the local blockchain state and the sequencer's (and underlying DA layer's) understanding of the blockchain.

Current Behavior

In the block package, when no batch is available from the sequencer (ErrNoBatch), the block manager creates empty blocks locally:

batchData, err := m.getTxsFromBatch()
if errors.Is(err, ErrNoBatch) {
    m.logger.Debug("No batch available, creating empty block")
    // Create an empty block instead of returning
    batchData = &BatchData{
        Batch: &coresequencer.Batch{Transactions: [][]byte{}},
        Time:  time.Now().Round(0).UTC(),
        Data:  [][]byte{},
    }
}

Problem

When a sequencer is being used, only batches provided by the sequencer should be used to create blocks. The current implementation allows the block manager to create empty blocks locally, which are not visible to the sequencer or the underlying DA layer, leading to divergent views of the blockchain.

Concrete Example

Consider a scenario with batches:

• B1, B2, B3, B4, B5: Batches returned by the sequencer
• E1, E2, E3: Empty batches created locally by Rollkit

If block production proceeds as follows: B1, B2, E1, B3, E2, E3, B4, B5, this results in 8 blocks produced locally. However, the sequencer (and the underlying DA layer) is only aware of 5 blocks (B1-B5).

Consequences

1. Inconsistent State: The local node will have a different blockchain state than what's represented on the DA layer.

2. Synchronization Issues: Other nodes attempting to sync will encounter:

   • Batch verification failures for empty batches when receiving blocks via P2P
   • Different total block counts and contents, resulting in different chain states

3. Time Inconsistency: There's no guarantee that batch times will be monotonically increasing, which may cause additional ordering issues.

Proposed Solution

When using a sequencer, the block manager should not create empty blocks locally. Instead, it should wait for batches from the sequencer, maintaining consistency between local state and the sequencer's view.
If empty blocks are required for liveness, this responsibility should be handled by the sequencer itself, ensuring all nodes have a consistent view of these empty blocks.

Additional Context

This issue affects consensus integrity when using a sequencer with Rollkit. The behavior is a remnant of the pre-sequencer design that needs to be updated to align with the current architecture.