fixup! [MIG] mail_gateway: continue migration

Author: chienandalu

mail_gateway/models/mail_thread.py

@@ -72,8 +72,8 @@ def _get_mail_thread_data(self, request_list):
         data = super()._get_mail_thread_data(request_list)
         data["gateway_followers"] = [
             f["partner"]
-            for f in data["followers"]
-            if data.get("followers") and f["partner"]["gateway_channels"]
+            for f in data.get("followers", [])
+            if f["partner"]["gateway_channels"]
         ]
         return data