fix npe when performing soroban reply without utxos

src/main/java/com/sparrowwallet/sparrow/soroban/SparrowCahootsWallet.java

@@ -15,6 +15,10 @@ import com.sparrowwallet.drongo.wallet.WalletNode;
 import com.sparrowwallet.sparrow.whirlpool.Whirlpool;
 import org.apache.commons.lang3.tuple.Pair;
 
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
 public class SparrowCahootsWallet extends SimpleCahootsWallet {
     private final Wallet wallet;
     private final int account;
@@ -39,6 +43,16 @@ public class SparrowCahootsWallet extends SimpleCahootsWallet {
         return account;
     }
 
+    @Override
+    protected List<CahootsUtxo> fetchUtxos(int account) {
+        List<CahootsUtxo> utxos = super.fetchUtxos(account);
+        if(utxos == null) {
+            utxos = new LinkedList<>();
+        }
+
+        return utxos;
+    }
+
     @Override
     public Pair<Integer, Integer> fetchReceiveIndex(int account) throws Exception {
         if(account == StandardAccount.WHIRLPOOL_POSTMIX.getAccountNumber()) {