fix(tasks): support change_interval before first loop run (#1291)

Author: shiftinv

changelog/1290.bugfix.rst

@@ -0,0 +1 @@
+|tasks| Correctly handle :meth:`Loop.change_interval <ext.tasks.Loop.change_interval>` when called from :meth:`~ext.tasks.Loop.before_loop` or through other means before initial loop run.

disnake/ext/tasks/__init__.py

@@ -689,7 +689,9 @@ def change_interval(
             self._time = self._get_time_parameter(time)
             self._sleep = self._seconds = self._minutes = self._hours = MISSING
 
-        if self.is_running():
+        # `_last_iteration` can be missing if `change_interval` gets called in `before_loop` or
+        # before the event loop ticks after `start()`
+        if self.is_running() and self._last_iteration is not MISSING:
             if self._time is not MISSING:
                 # prepare the next time index starting from after the last iteration
                 self._prepare_time_index(now=self._last_iteration)