[web] Make ChatMessageList able to fetch messages if none are present

Summary:
With the changes in the earlier diff, we may end up with zero messages for a given thread. This diff makes it possible for us to fetch messages in that case.

Depends On D3350

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: palys-swm

Subscribers: benschac, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3351