checks for null data transaction before debug.log

CWalletTx::GetAmounts could not find output address for null data transactions, thus issuing an error in debug.log. This change checks to see if the transaction is OP_RETURN before issuing error.

resolves #6142

src/wallet/wallet.cpp

@@ -1034,7 +1034,8 @@ void CWalletTx::GetAmounts(list<COutputEntry>& listReceived,
 
         // In either case, we need to get the destination address
         CTxDestination address;
-        if (!ExtractDestination(txout.scriptPubKey, address))
+
+        if (!ExtractDestination(txout.scriptPubKey, address) && txout.scriptPubKey[0] != OP_RETURN)
         {
             LogPrintf("CWalletTx::GetAmounts: Unknown transaction type found, txid %s\n",
                      this->GetHash().ToString());