Set ASSUME_NONULL in codegen files (#47481)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47481

The Codegenerated files has this option turned off.
This is causing Xcode to output hundreds of warnings due to missing nullability options. RThis fixes them.

## CHANGELOG
[Internal] - Set ASSUME_NONNULL regions in codegen'd files

Reviewed By: dmytrorykun

Differential Revision: D65596598

fbshipit-source-id: bbf664944e103c05ef593a7e07bf5b767445950c

packages/react-native/scripts/codegen/generate-specs-cli-executor.js

@@ -80,6 +80,7 @@ function generateSpecFromInMemorySchema(
       schema,
       outputDirectory,
       packageName,
+      assumeNonnull: platform === 'ios',
       useLocalIncludePaths,
     },
     {