Conpherence - improve stack re: non-update updates

Summary:
Fixes T7761. Fixes T7318.

When we send an empty message to the server, pretend its just a request to load the page. Make load a bit smarter such that if we don't get back any transactions, rather than error like the fool, just send down to the client the notion of a 'non_update'. Instrument the client to just turn off the appropriate loading state, etc for a non update.

T7318 is a tricky beast since we don't know exactly how to reproduce it but if / when it occurs again it would be some other bizarre application behavior maybe? We won't be getting the execption anymore, that's for sure.

Test Plan: removed code in ConpherenceThreadManager.sendMessage that protects against sending empty messages. sent empty messages (non updates) like whoa and everything worked on both durable column and main column view. re-added the code in ConpherenceThreadManager.sendMessage and noted empty messages did not send while any text including a space sent up nicely

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7318, T7761

Differential Revision: https://secure.phabricator.com/D12339