navReducer: Better decide if we have server data in rehydrate.

As hinted by the comment above this edit, even an empty array is
incomplete server data: "Valid server data must have a user: the
self user, at a minimum.".

But the initial state of `users` (`initialState` in
src/users/usersReducer.js) is an empty array. LOGOUT, LOGIN_SUCCESS,
and ACCOUNT_SWITCH all set `users` to that initial state.

Those three actions are handled in `navReducer` by navigating to
routes that don't depend on complete server data: 'loading' or
'account'.

But if we quit the app with `users` having this initial state (e.g.,
giving up in frustration while trying to connect to a realm for a
long time), we should be sure that, on reopening it and rehydrating,
we don't try to go to 'main' before that initial state has been
replaced with a complete state. Otherwise, we see "No server data
found" errors and a white screen.

See also discussion [1].

[1]: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M4166.20Time.20out.20initial.20fetch/near/912017

src/nav/navReducer.js

@@ -61,7 +61,7 @@ const rehydrate = (state, action) => {
   // will have set `needsInitialFetch`, too, so we really will be loading.
   //
   // (Valid server data must have a user: the self user, at a minimum.)
-  if (rehydratedState.users === undefined) {
+  if (rehydratedState.users === undefined || rehydratedState.users.length === 0) {
     return getStateForRoute('loading');
   }