message: rpc/wallet: use OMITTED_NAMED_ARG instead of Default(VNULL)
diff: @@ -2562,7 +2562,7 @@ static RPCHelpMan loadwallet()
  "\napplied to the new wallet (eg -rescan, etc).\n",
  {
  {"filename", RPCArg::Type::STR, RPCArg::Optional::NO, "The wallet directory or .dat file."},
  - {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Default{UniValue::VNULL}, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  + {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Optional::OMITTED_NAMED_ARG, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  },
  RPCResult{
  RPCResult::Type::OBJ, "", "",
  @@ -2693,7 +2693,7 @@ static RPCHelpMan createwallet()
  {"passphrase", RPCArg::Type::STR, RPCArg::Optional::OMITTED_NAMED_ARG, "Encrypt the wallet with this passphrase."},
  {"avoid_reuse", RPCArg::Type::BOOL, RPCArg::Default{false}, "Keep track of coin reuse, and treat dirty and clean coins differently with privacy considerations in mind."},
  {"descriptors", RPCArg::Type::BOOL, RPCArg::Default{false}, "Create a native descriptor wallet. The wallet will use descriptors internally to handle address creation"},
  - {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Default{UniValue::VNULL}, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  + {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Optional::OMITTED_NAMED_ARG, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  {"external_signer", RPCArg::Type::BOOL, RPCArg::Default{false}, "Use an external signer such as a hardware wallet. Requires -signer to be configured. Wallet creation will fail if keys cannot be fetched. Requires disable_private_keys and descriptors set to true."},
  },
  RPCResult{
  @@ -2784,7 +2784,7 @@ static RPCHelpMan unloadwallet()
  "Specifying the wallet name on a wallet endpoint is invalid.",
  {
  {"wallet_name", RPCArg::Type::STR, RPCArg::DefaultHint{"the wallet name from the RPC endpoint"}, "The name of the wallet to unload. If provided both here and in the RPC endpoint, the two must be identical."},
  - {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Default{UniValue::VNULL}, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  + {"load_on_startup", RPCArg::Type::BOOL, RPCArg::Optional::OMITTED_NAMED_ARG, "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},
  },
  RPCResult{RPCResult::Type::OBJ, "", "", {
  {RPCResult::Type::STR, "warning", "Warning message if wallet was not unloaded cleanly."},
  