RPC eth_getTransactionByBlockNumberAndIndex does not work as expected when querying a cosmos transaction

[banishee]

System info: [Include Ethermint commit, operating system name, and other relevant details]

Steps to reproduce:

1. Assuming the transaction index 9 is belong to a cosmos transaction, calls this api with such a parameter:

params: [
   '0x29c', //668
   '0x9' // 9
]

2. the result will be null

Expected behavior: [What you expected to happen]

• It is supposed to return a transaction object in any condition, as long as it receives the legal parameter.

Actual behavior: [What actually happened]

• It returns null when the index does not belong to MsgEthereumTx transaction

Additional info: [Include gist of relevant config, logs, etc.]

• linked to #808

[github-actions[bot]]

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