Fix PSBT deserialization of 0-input transactions

0-input transactions can be ambiguously deserialized as being witness
transactions. Since the unsigned transaction is never serialized as
a witness transaction as it has no witnesses, we should always
deserialize it as a non-witness transaction and set the serialization
flags as such.

Also always serialize the unsigned transaction as a non-witness transaction.

src/streams.h

@@ -61,6 +61,7 @@ public:
 
     int GetVersion() const { return nVersion; }
     int GetType() const { return nType; }
+    size_t size() const { return stream->size(); }
 };
 
 template<typename S>