InitialNavigationDispatcher: Clarify and improve doInitialNavigation.

Staring at the code before this change, it's not clear why we'd want
to go to the realm-input screen in the case where we know about
exactly one account. That's the place we go when we don't have any
idea what account the user might want to log in as.

I suppose one might say it's weird to land on the account-picker
screen when we only know about one account -- but we know that the
account isn't logged in, so we can't go ahead and show its data. It
also helps the user get where they're going faster by giving them
something to tap on, which will take them to the auth screen to log
in.

See discussion [1].

[1] zulip#4274 (comment)

Author: chrisbobbe

src/nav/InitialNavigationDispatcher.js

@@ -43,15 +43,16 @@ class InitialNavigationDispatcher extends PureComponent<Props> {
   doInitialNavigation = () => {
     const { hasAuth, accounts, haveServerData, dispatch } = this.props;
 
+    // If we don't know about any accounts, go to the realm-input screen.
+    if (accounts.length === 0) {
+      dispatch(resetToRealmScreen({ initial: true }));
+      return;
+    }
+
     // If the active account is not logged in, show account screen.
     if (!hasAuth) {
-      if (accounts.length > 1) {
-        dispatch(resetToAccountPicker());
-        return;
-      } else {
-        dispatch(resetToRealmScreen({ initial: true }));
-        return;
-      }
+      dispatch(resetToAccountPicker());
+      return;
     }
 
     // If there's an active, logged-in account but no server data, then behave