Modify native module codegen to throw JS errors when passing null values for non-nullable arguments

Summary:
Changelog: [General][Breaking] Native modules using the codegen now throw an error when called with `null` for optional but not nullable arguments.

## Context

Right now, if you have a native module using the codegen with a method like this:

```
someMethod(value?: number): void;
```

And you call it like this:

```
NativeModule.someMethod(null);
```

The app doesn't throw an error, but it should because this method shouldn't accept `null` according to its type definition.

## Changes

This modifies the codegen to only check for `undefined` in those cases, otherwise trying to cast the value to the expected type and failing if it's `null`.

NOTE: this is technically a breaking change, but if people are using Flow or TypeScript in their projects they're very unlikely to hit this case, because they would've complained if you tried to pass `null` in these cases.

Reviewed By: cipolleschi

Differential Revision: D54206289

fbshipit-source-id: 58f2f2f3009d203b96189d3c66d1ae98a9e4fb36

packages/react-native-codegen/src/generators/modules/GenerateModuleCpp.js

@@ -132,6 +132,12 @@ function serializeArg(
     const val = `args[${index}]`;
     const expression = callback(val);
 
+    // param?: T
+    if (optional && !nullable) {
+      // throw new Error('are we hitting this case? ' + moduleName);
+      return `count <= ${index} || ${val}.isUndefined() ? std::nullopt : std::make_optional(${expression})`;
+    }
+
     // param: ?T
     // param?: ?T
     if (nullable || optional) {