Fix push for m.read events (#12721)

badge_count_last_call was always zero when the response for push
notifications included a "rejected" key which mapped to an empty list.

changelog.d/12721.bugfix

+Fix push to dismiss notifications when read on another client. Contributed by @SpiritCroc @ Beeper.

synapse/push/httppusher.py

@@ -405,7 +405,7 @@ class HttpPusher(Pusher):
         rejected = []
         if "rejected" in resp:
             rejected = resp["rejected"]
-        else:
+        if not rejected:
             self.badge_count_last_call = badge
         return rejected