Avoid panic when push messages are empty or nil (#13751)

i18n/en.json

@@ -1762,10 +1762,18 @@
     "id": "api.push_notification.id_loaded.fetch.app_error",
     "translation": "An error occurred fetching the ID-loaded push notification"
   },
+  {
+    "id": "api.push_notifications.message.parse.app_error",
+    "translation": "An error occurred building the push notification message"
+  },
   {
     "id": "api.push_notifications_ack.forward.app_error",
     "translation": "An error occurred sending the receipt delivery to the push notification service"
   },
+  {
+    "id": "api.push_notifications_ack.message.parse.app_error",
+    "translation": "An error occurred building the push notification ack message"
+  },
   {
     "id": "api.reaction.delete.archived_channel.app_error",
     "translation": "You cannot remove a reaction in an archived channel."