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