This internal utility function is used to clean up the cov environment by removing all objects and optionally displaying a message before returning `NULL`. It is primarily used within `run_coverage_base()` to ensure consistent cleanup in error-handling scenarios.
cleanup_and_return_null(msg = NULL, env = cov_env)NULL (invisibly)
Optional character string. If provided, the message will be printed before cleanup.
A character string specifying the covr env