Change the way no_updates mode is enabled

See discussion on https://github.com/LonamiWebs/Telethon/commit/49713b2.

The problem with the automatic approach is that some scripts may do
some "fancier" things with the way they register updates, so it was
prone to failure (a handler could be added but since the last request
was without updates, nothing would be received).

This new approach is a bit more annoying to opt-into but also more
explicit.

telethon/client/users.py

@@ -51,7 +51,7 @@ class UserMethods:
                 else:
                     raise errors.FloodWaitError(request=r, capture=diff)
 
-            if not self._event_builders and not self._conversations:
+            if self._no_updates:
                 r = functions.InvokeWithoutUpdatesRequest(r)
 
         request_index = 0