__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: ```shell params: [ '0xe670ec64341771606e55d6b4ca35a1a6b75ee3d5145a99d05921026d1527331', '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 #809
System info: [Include Ethermint commit, operating system name, and other relevant details]
Steps to reproduce:
9is belong to a cosmos transaction, calls this api with such a parameter:nullExpected behavior: [What you expected to happen]
Actual behavior: [What actually happened]
nullwhen the index does not belong toMsgEthereumTxtransactionAdditional info: [Include gist of relevant config, logs, etc.]