Crash on attempting @-mention starting with "a" or "e"

[gnprice]

This repro works on v27.158 (the current beta) and I believe on current master as well:

• Go to a compose box.
• Type @a, or @e.
• The app's UI disappears and the app just shows a white screen.

The culprit is the fake user-like data that our autocomplete code generates for the @all and @everyone mentions, and that data's interaction with UserAvatarWithPresenceById, which is passed a user ID and expects that user ID to belong to an actual user.

Instead, when we're handling such fake data, we should simply return an empty React element like null in place of the avatar.

(Also we can probably do better than writing this in terms of fake pseudo-users in the first place. But that's a bigger refactor.)

[gnprice]

I'm looking through Sentry issues that were new in v27.158, the current beta, and the most widespread is this one:
https://sentry.io/organizations/zulip/issues/2143370130/events/f029e943dff343d4a69031505da47556

getUserForId: missing user: id -1

with a stack of getUserForId < UserAvatarWithPresenceById < React renderer.

I believe that is a symptom of this bug.