Remove null char from contract data asset_code in avro files

[chowbao]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with the jira ticket associated with the PR.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated the README with the added features, breaking changes, new instructions on how to use the repository.

What

Strip out the null chars \u0000 from the asset_code column in contract_data

Why

Dune found that asset_code had null chars appended for some asset_codes which should be removed
https://stellarfoundation.slack.com/archives/C07N40J1789/p1728568351934499?thread_ts=1727985059.719139&cid=C07N40J1789

Known limitations

• This is just a patch to get Dune unblocked for now

I believe this is caused because of the way asset_code in contracts are saved as ScVal and reconstructed to a string in stellar-etl. It seems like the null chars appear in asset_codes of type alpha12.

Next week we should:

• Update stellar-etl to strip the ScVal --> string asset_code from contract_data
  • strip null chars from asset code in contract data stellar-etl#285
• Strip the null chars from asset_code in contract_data in our BQ tables