fix: not recursively camelCase keys in nested objects when using union with null

[yutak23]

This ought to fix #118.

The bug in the issue is due to the fact that the type ObjectOptional assumed only undefined (optional property), so I have fixed that part.

Supplemental information on testing

Since the type CamelCaseKeys cannot be tested as is, it is tested by specifying the type CamelCaseKeys for the variable after conversion with camelcaseKeys.

The test for index.d.ts before the modification failed as follows, but after the modification, the test passes because the union part works correctly.