feat(pytests): OmmersRoot is EmptyOmmersRoot by default instead of 0

[winsvega]

🗒️ Description

We see this check done by ethereumjs t8n that actually after PoS, ommers root is required to be a has of empty list.
But pyspec passes 0 padded hash by default. only ethereumjs does this check. this seems to only be related to ParentUncleHash field.

🔗 Related Issues

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: All converted JSON/YML tests from ethereum/tests have been added to converted-ethereum-tests.txt.
• Tests: A PR with removal of converted JSON/YML tests from ethereum/tests have been opened.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[marioevz]

LGTM, thanks!