Avoid error when trying to reset deadline for no msgbox entries

Closes #4520.

telethon/_updates/messagebox.py

@@ -295,6 +295,8 @@ class MessageBox:
     #
     # It also updates the next deadline time to reflect the new closest deadline.
     def reset_deadlines(self, entries, deadline):
+        if not entries:
+            return
         for entry in entries:
             if entry not in self.map:
                 raise RuntimeError('Called reset_deadline on an entry for which we do not have state')