[lib] Show loader when there are no messages

Summary:
Earlier changes in this stack made it possible for a thread to appear without any messages. Without this change, the loader doesn't appear when there aren't messages, which means that MessageList in native never triggers a message fetch.

Depends On D3352

Test Plan:
I decreased defaultMaxMessageAge such that no messages got loaded at all, and made sure that when I opened a thread without messages, the messages were fetche
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Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: benschac, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3353