The REQUEST.rawBody is the same as the lambda event body which should be string | null but is defined as string in the type definition.
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/trigger/api-gateway-proxy.d.ts#L117
https://github.com/jeremydaly/lambda-api/blob/main/index.d.ts#L146
Can it be updated to match? I'm happy to raise pull request.