rpcwallet: Replace boost::optional<T>::emplace with simple assignment of T{}

Optional::emplace() was only added in boost 1.56, see
boostorg/optional@2e583aa

To simply work around bitcoin/bitcoin#18943,
replace it with assignment of T{}

Author: MarcoFalke

src/wallet/rpcwallet.cpp

@@ -1557,8 +1557,8 @@ static UniValue listsinceblock(const JSONRPCRequest& request)
     uint256 blockId;
     if (!request.params[0].isNull() && !request.params[0].get_str().empty()) {
         blockId = ParseHashV(request.params[0], "blockhash");
-        height.emplace();
-        altheight.emplace();
+        height = int{};
+        altheight = int{};
         if (!pwallet->chain().findCommonAncestor(blockId, pwallet->GetLastBlockHash(), /* ancestor out */ FoundBlock().height(*height), /* blockId out */ FoundBlock().height(*altheight))) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
         }