Use GetUnsignedTx when serializing in PSBTv0

If we are asked to make a PSBTv0, we may not necessarily have made an
unsigned transaction. So instead use GetUnsignedTx which will either
fetch one that already exists, or construct a new one from the stored
data. Internally we may be storing a PSBTv0 like a PSBTv2, but still
want to serialize those as v0.

src/psbt.h

@@ -761,7 +761,7 @@ struct PartiallySignedTransaction
 
             // Write serialized tx to a stream
             OverrideStream<Stream> os(&s, s.GetType(), s.GetVersion() | SERIALIZE_TRANSACTION_NO_WITNESS);
-            SerializeToVector(os, *tx);
+            SerializeToVector(os, GetUnsignedTx());
         }
 
         // Write xpubs