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

@@ -1345,7 +1345,7 @@ struct PartiallySignedTransaction
             SerializeToVector(s, CompactSizeWriter(PSBT_GLOBAL_UNSIGNED_TX));
 
             // Write serialized tx to a stream
-            SerializeToVector(s, TX_NO_WITNESS(*tx));
+            SerializeToVector(s, TX_NO_WITNESS(GetUnsignedTx()));
         }
 
         // Write xpubs