TL;DR
This (most likely) means you've discovered and are attempting to get info from a Sonos bridge, which does not include any of the other information at the currently retrieved URL.

In my (limited) experience, a missing ZoneName means you are hitting a device that has _no_ information at the .../zp url (e.g. a bridge) - it may be possible to get some of the other info from the bridge's .../lp url, but it does not contain all of the same information, and it's not likely we'd want to do anything (currently) from SoCo to a bridge device