Allow sending MPP keysend

This means allowing a user to send an MPP without a payment secret if it's a
keysend payment.

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -8351,7 +8351,7 @@ mod tests {
 
 		match nodes[0].node.send_payment(&route, payment_hash, &None, PaymentId(payment_hash.0)).unwrap_err() {
 			PaymentSendFailure::ParameterError(APIError::APIMisuseError { ref err }) => {
-				assert!(regex::Regex::new(r"Payment secret is required for multi-path payments").unwrap().is_match(err))
+				assert!(regex::Regex::new(r"Payment secret is required for non-keysend multi-path payments").unwrap().is_match(err))
 			},
 			_ => panic!("unexpected error")
 		}

lightning/src/ln/outbound_payment.rs

@@ -892,8 +892,8 @@ impl OutboundPayments {
 		if route.paths.len() < 1 {
 			return Err(PaymentSendFailure::ParameterError(APIError::InvalidRoute{err: "There must be at least one path to send over".to_owned()}));
 		}
-		if payment_secret.is_none() && route.paths.len() > 1 {
-			return Err(PaymentSendFailure::ParameterError(APIError::APIMisuseError{err: "Payment secret is required for multi-path payments".to_owned()}));
+		if payment_secret.is_none() && route.paths.len() > 1 && keysend_preimage.is_none() {
+			return Err(PaymentSendFailure::ParameterError(APIError::APIMisuseError{err: "Payment secret is required for non-keysend multi-path payments".to_owned()}));
 		}
 		let mut total_value = 0;
 		let our_node_id = node_signer.get_node_id(Recipient::Node).unwrap(); // TODO no unwrap