Fix actually filtering spent utxos out

Author: nielstron

pycardano/backend/ogmios.py

@@ -267,7 +267,7 @@ def _utxos_kupo(self, address: str) -> List[UTxO]:
             tx_id = result["transaction_id"]
             index = result["output_index"]
 
-            if result["spent_at"] is not None:
+            if result["spent_at"] is None:
                 tx_in = TransactionInput.from_primitive([tx_id, index])
 
                 lovelace_amount = result["value"]["coins"]