Reject whitespace-only cron delivery target strings before cron input normalization can trim and drop them, so bad delivery targets return INVALID_REQUEST instead of behaving as omitted fields.
Keep explicit null update clears for delivery, failure destination, and completion destination fields.
Co-authored-by: gaozixiang1 <gaozixiang1@xiaomi.com>
Co-authored-by: Lanzhi <lizhan3@xiaomi.com>