Fix off-by-one error in cxx codegen (#36574)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36574

We would previously generate the following codegen for optional args

```
return static_cast<NativeAudioModuleCxxSpecJSI *>(&turboModule)->playAudio(
    rt,
    args[0].asString(rt),
    args[1].isNull() || args[1].isUndefined() ? std::nullopt : std::make_optional(args[1].asString(rt)),
    args[2].isNull() || args[2].isUndefined() ? std::nullopt : std::make_optional(args[2].asString(rt)),
    args[3].asNumber(),
    count < 4 || args[4].isNull() || args[4].isUndefined() ? std::nullopt : std::make_optional(args[4].asObject(rt)),
    count < 5 || args[5].isNull() || args[5].isUndefined() ? std::nullopt : std::make_optional(args[5].asObject(rt)),
    count < 6 || args[6].isNull() || args[6].isUndefined() ? std::nullopt : std::make_optional(args[6].asBool())
);
```

However, the counts checked are off-by-one, causing us to incorrectly process args.

Changelog: [General][Fixed] Issue with TurboModule C++ codegen with optional args

Differential Revision: D44299193

fbshipit-source-id: f00b9f5e09c2f524f9393137346c256d8b6b2979

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.cpp

@@ -117,6 +117,12 @@ AsyncPromise<std::string> NativeCxxModuleExample::getValueWithPromise(
   return promise;
 }
 
+bool NativeCxxModuleExample::getWithWithOptionalArgs(
+    jsi::Runtime &rt,
+    std::optional<bool> optionalArg) {
+  return optionalArg.value_or(false);
+}
+
 void NativeCxxModuleExample::voidFunc(jsi::Runtime &rt) {
   // Nothing to do
 }