[tasks] remove redundant condition in Loop.next_iteration

self._task is only None if the Loop has never been started before, 
which means None should be returned always, regardless of how
many seconds was passed into the constructor

this didn't break anything before because self._next_iteration will
be None as well if self._task is None.

discord/ext/tasks/__init__.py

@@ -154,7 +154,7 @@ class Loop:
 
         .. versionadded:: 1.3
         """
-        if self._task is None and self._sleep:
+        if self._task is None:
             return None
         elif self._task and self._task.done() or self._stop_next_iteration:
             return None