Fix explorer panic on last page

Motivation

We have a bug when navigating the last pages of batches on https://stage.explorer.alignedlayer.com/batches and https://holesky.explorer.alignedlayer.com/batches.

This occurs because the old batches don't have a fee_per_proof field in the db. So it panics when calling EthConverter.wei_to_eth() with a nil value.

Description

Handles properly the case where fee_per_proof is nil.

Type of change

• Bug fix

How to test

Steps to reproduce the bug locally

1. git checkout testnet
2. Run a local net with the explorer and send some batches.
3. Enter to the explorer db (postgresql container) edit the fee_per_proof of one batch:

docker exec -it <container_id> bash`
psql -U explorer_user -d explorer_db

UPDATE batches SET fee_per_proof = NULL WHERE merkle_root = '0xfc97db8cc6bd3e838b9924fc278ee20620374707ad89e81406f23dad97fd8ae3'
RETURNING *;

4. Go to localhost:4000/batches and navigate through the pages until the edited batch. You should see the panic
5. You should also see the panic going to localhost:4000/batches/0xfc97db8cc6bd3e838b9924fc278ee20620374707ad89e81406f23dad97fd8ae3 with the chosen merkle root.

Test the fix

1. git checkout hotfix/explorer_last_page
2. Run the local net and navigate through localhost:4000/batches and localhost:4000/batches/0xfc97db8cc6bd3e838b9924fc278ee20620374707ad89e81406f23dad97fd8ae3. The panic shouldn't appear.
3. fee_per_proof should look like this:

Checklist

• “Hotfix” to testnet, everything else to staging
• Linked to Github Issue
• This change depends on code or research by an external entity
  • Acknowledgements were updated to give credit
• Unit tests added
• This change requires new documentation.
  • Documentation has been added/updated.
• This change is an Optimization
  • Benchmarks added/run
• Has a known issue
  • Link to the open issue addressing it
• If your PR changes the Operator compatibility (Ex: Upgrade prover versions)
  • This PR adds compatibility for operator for both versions and do not change batcher/docs/examples
  • This PR updates batcher and docs/examples to the newer version. This requires the operator are already updated to be compatible