Change default note contents in Tx builder and cheat message change

src/transaction_builder.h

@@ -89,7 +89,7 @@ public:
         uint256 ovk,
         libzcash::SaplingPaymentAddress to,
         CAmount value,
-        std::array<unsigned char, ZC_MEMO_SIZE> memo = {{0xF6}});
+        std::array<unsigned char, ZC_MEMO_SIZE> memo = {{0}});
 
     // Assumes that the value correctly corresponds to the provided UTXO.
     void AddTransparentInput(COutPoint utxo, CScript scriptPubKey, CAmount value);