Hotfix explorer

Description

The explorer had errors when started with no proofs.
One is it tried to make log10(0) when no proofs where present.
Another one is the cost_per_proof_chart var name was not updated in set_empty_values()

To test

Start an explorer with no proofs, make sure it starts with no errors.
Send some proofs, make sure it works as expected

Type of change

Please delete options that are not relevant.

• New feature
• Bug fix
• Optimization
• Refactor

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