Fix validation of network names in Almanac

Summary: Ref T5833. This was using the wrong constant, so we weren't validating property.

Test Plan: Tried to create a nameless network and correctly got an error.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D11447