Verizon's policy is to collect a signature before delivery and by failing to do so, the responsibility falls 100% on FedEx.
If they want to fight you on it or deny responsibility, show them this: https://www.verizon.com/support/shipping-faqs/
This is assumimg what you ordered was a phone or another piece of equipment valued at $75 or above. If your package fits under that, FedEx failed to do their job and cannot place blame on the shipper for an empty box. Had a signature been collected and the box was empty, then we can put the blame on Verizon.