wallet: use optional for change position as an optional in CreateTransaction

Instead of making -1 a magic number meaning no change or random change
position, use an optional to have that meaning.

src/wallet/test/spend_tests.cpp

@@ -28,13 +28,12 @@ BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
     // instead of the miner.
     auto check_tx = [&wallet](CAmount leftover_input_amount) {
         CRecipient recipient{PubKeyDestination({}), 50 * COIN - leftover_input_amount, /*subtract_fee=*/true};
-        constexpr int RANDOM_CHANGE_POSITION = -1;
         CCoinControl coin_control;
         coin_control.m_feerate.emplace(10000);
         coin_control.fOverrideFeeRate = true;
         // We need to use a change type with high cost of change so that the leftover amount will be dropped to fee instead of added as a change output
         coin_control.m_change_type = OutputType::LEGACY;
-        auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, coin_control);
+        auto res = CreateTransaction(*wallet, {recipient}, std::nullopt, coin_control);
         BOOST_CHECK(res);
         const auto& txr = *res;
         BOOST_CHECK_EQUAL(txr.tx->vout.size(), 1);

src/wallet/test/wallet_tests.cpp

@@ -558,8 +558,7 @@ public:
         CTransactionRef tx;
         CCoinControl dummy;
         {
-            constexpr int RANDOM_CHANGE_POSITION = -1;
-            auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, dummy);
+            auto res = CreateTransaction(*wallet, {recipient}, std::nullopt, dummy);
             BOOST_CHECK(res);
             tx = res->tx;
         }