fix(eth): serialize empty lists as [] instead of null

[sudo-shashank]

Summary of changes

Changes introduced in this pull request:

• Use NotNullVec instead of Vec for empty lists so they serialize as [] instead of null.

Reference issue to close (if applicable)

Closes #7205

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
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Summary by CodeRabbit

Bug Fixes

• Fixed Ethereum JSON-RPC methods to serialize empty lists as [] instead of null. Affected methods include eth_accounts, eth_getBlockReceipts, trace_block, trace_transaction, and related trace endpoints.

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