Attempting to get non existing unconfirmed payment by tx_id throws error
When retrieving a non existing unconfirmed payment by its tx_id, `wallet.incoming` throws an error instead of returning an empty array.

Example:
```
>>> wallet.incoming(tx_id='non existing tx_id', unconfirmed=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/dist-packages/monero/transaction/__init__.py", line 313, in __call__
    return fetch(self.account_idx, PaymentFilter(**filterparams))
  File "/usr/local/lib/python3.8/dist-packages/monero/backends/jsonrpc/wallet.py", line 178, in transfers_in
    pmts.extend(_pmts.get("pool", []))
UnboundLocalError: local variable '_pmts' referenced before assignment
```