fix: reject empty pre-sign requests (#1190)

* Reject basically empty bitcoin transaction pre-sign requests
* Add more empty request checks

signer/src/bitcoin/utxo.rs

@@ -798,7 +798,7 @@ impl<'a> UnsignedTransaction<'a> {
     /// Construct an unsigned transaction.
     ///
     /// This function can fail if the output amounts are greater than the
-    /// input amounts.
+    /// input amounts or if the [`Requests`] object is empty.
     ///
     /// The returned BTC transaction has the following properties:
     ///   1. The amounts for each output has taken fees into consideration.
@@ -822,7 +822,7 @@ impl<'a> UnsignedTransaction<'a> {
     /// Construct a transaction with stub witness data.
     ///
     /// This function can fail if the output amounts are greater than the
-    /// input amounts.
+    /// input amounts or if the [`Requests`] object is empty.
     ///
     /// The returned BTC transaction has the following properties:
     ///   1. The amounts for each output has taken fees into consideration.
@@ -833,6 +833,9 @@ impl<'a> UnsignedTransaction<'a> {
     ///   5. All witness data is correctly set, except for the fake
     ///      signatures from (4).
     pub fn new_stub(requests: Requests<'a>, state: &SignerBtcState) -> Result<Self, Error> {
+        if requests.is_empty() {
+            return Err(Error::BitcoinNoRequests);
+        }
         // Construct a transaction base. This transaction's inputs have
         // witness data with dummy signatures so that our virtual size
         // estimates are accurate. Later we will update the fees.
@@ -1806,10 +1809,9 @@ mod tests {
         assert_eq!(new_utxo.public_key, requests.signer_state.public_key);
     }
 
-    /// Transactions that do not service requests do not have the OP_RETURN
-    /// output.
+    /// You cannot create sweep transactions that do not service requests.
     #[test]
-    fn no_requests_no_op_return() {
+    fn no_requests_no_sweep() {
         let public_key = XOnlyPublicKey::from_str(X_ONLY_PUBLIC_KEY1).unwrap();
         let signer_state = SignerBtcState {
             utxo: SignerUtxo {
@@ -1823,14 +1825,9 @@ mod tests {
             magic_bytes: [0; 2],
         };
 
-        // No requests so the first output should be the signers UTXO and
-        // that's it. No OP_RETURN output.
         let requests = Requests::new(Vec::new());
-        let unsigned = UnsignedTransaction::new(requests, &signer_state).unwrap();
-        assert_eq!(unsigned.tx.output.len(), 1);
-
-        // There is only one output and it's a P2TR output
-        assert!(unsigned.tx.output[0].script_pubkey.is_p2tr());
+        let sweep = UnsignedTransaction::new(requests, &signer_state);
+        assert!(sweep.is_err());
     }
 
     /// We aggregate the bitmaps to form a single one at the end. Check