[Mt-batcher]: mt-batcher pushes one less block

[HXHke]

Describe the bug

1. When mt-batch pushes the batch to da, da will process less of the last block, resulting in one less block of data in this batch. The data of this block will not be integrated into da until the next batch.
2. If there is no batch, the currentL2TransactionIndex metric will decrease, which is theoretically not possible. It is a counter type Prometheus monitoring.

Screenshots

1. Mt-batcher n/a
2. 

How to deal

1. Mt-batcher adds a block when processing each batch.
2. This metric is only pushed when the batch exists.