Fix panic with empty transaction arguments (#632)

Author: jribbink

flow.go

@@ -162,7 +162,7 @@ func mustRLPEncode(v interface{}) []byte {
 	// Note(sideninja): This is a temporary workaround until cadence defines canonical format addressing the issue https://github.com/onflow/flow-go-sdk/issues/286
 	if tx, ok := v.(payloadCanonicalForm); ok {
 		for _, arg := range tx.Arguments {
-			if arg[len(arg)-1] == byte(10) {
+			if len(arg) > 0 && arg[len(arg)-1] == byte(10) {
 				arg = arg[:len(tx.Arguments)-1]
 			}
 		}

transaction.go

@@ -380,7 +380,7 @@ func (t *Transaction) payloadCanonicalForm() payloadCanonicalForm {
 
 	// note(sideninja): This is a temporary workaround until cadence defines canonical format addressing the issue https://github.com/onflow/flow-go-sdk/issues/286
 	for i, arg := range t.Arguments {
-		if arg[len(arg)-1] == byte(10) { // extra new line character
+		if len(arg) > 0 && arg[len(arg)-1] == byte(10) { // extra new line character
 			t.Arguments[i] = arg[:len(arg)-1]
 		}
 	}