push_notifications: Skip enqueuing notif for users without push devices.

Previously, we enqueued events to "missedmessage_mobile_notifications"
even for users who had no registered push devices.
'handle_push_notification' later used to perform the check & skip
if there were no registered devices.

This commit avoids putting such events into the
"missedmessage_mobile_notifications" queue at all. By doing so,
we reduce unnecessary churn.

zerver/lib/test_classes.py

@@ -1917,6 +1917,23 @@ Output:
             realm, licenses, licenses_at_next_renewal, CustomerPlan.BILLING_SCHEDULE_MONTHLY
         )
 
+    def register_push_device(self, user_profile_id: int) -> None:
+        PushDevice.objects.create(
+            user_id=user_profile_id,
+            push_account_id=10,
+            bouncer_device_id=1,
+            token_kind=PushDevice.TokenKind.FCM,
+            push_public_key="n4WTVqj8KH6u0vScRycR4TqRaHhFeJ0POvMb8LCu8iI=",
+        )
+
+    def register_push_device_token(self, user_profile_id: int) -> None:
+        PushDeviceToken.objects.create(
+            user_id=user_profile_id,
+            kind=PushDeviceToken.APNS,
+            token="test-token",
+            ios_app_id="com.zulip.flutter",
+        )
+
     def create_user_notifications_data_object(
         self, *, user_id: int, **kwargs: Any
     ) -> UserMessageNotificationsData: