msglist: When the initial message fetch comes up empty, auto-focus the compose box

[chrisbobbe]

Part of #127 (see below).

Normally, it would be bothersome if we auto-focused the compose box when opening a message-list page, because the keyboard takes up a lot of space when it appears, and that gets in the way of reading messages. Opening the keyboard isn't a problem when there are no messages, though, and it can actually be quite helpful—especially when opening a new DM narrow with no history.

Implementing this feature would address item 6 in Vlad's feedback about the new-DM UI:

• new_dm: Add UI for starting new DM conversations #1322 (comment)
• #mobile > Flutter DMs flow design review @ 💬

6. Keyboard should be opened once transitioned to compose dialog.

It is ok to close keyboard if user clicked outside and input is empty, but by default keyboard should be opened and input is focused for me to type a message