Error when trying to access the mempool

### Request
`curl --header "Content-Type: application/json" --request POST --data '{"IsMempool":true}' http://localhost:17001/api/v1/transaction-info`

### Response
`{"B1KKKiGg":"UpdateTxindex: Error connecting txn for mempool request: \u003c TxHash: 1cb76d55c2c33e94411c6fc83109d0fc1efc286e0d451f790d888238dbdf4236, TxnType: BC1YLfgstADdBR2TPmY4njYCLafatVibpFn4hGRX2i8zDs34X2pQhqV, PubKey: PRIVATE_MESSAGE \u003e: UpdateTxindex: Error connecting txn to UtxoView: ConnectTransaction: : _connectPrivateMessage: : RuleErrorInputSpendsNonexistentUtxo"}`

### What i expect
If you go to the admin panel on the node and the mempool section, you can see that there are statistics, but when receiving data from the mempool, there is an incomprehensible error. In response, I expect to receive the same information as on the explorer.bitclout.com

Start by reproducing the curl POST request to /api/v1/transaction-info with IsMempool enabled, then compare its response with the node admin panel's mempool statistics and explorer.bitclout.com. Trace the transaction-info API and mempool handling until the RuleErrorInputSpendsNonexistentUtxo response is located; done means the endpoint returns the expected mempool information instead of the error.

Written by the indexing model from the issue text.