Empty sync response could be more empty

[bmarty]

Currently an empty sync response (i.e. without any change) is like that (with formatting added):

{
  "account_data": {
    "events": []
  },
  "to_device": {
    "events": []
  },
  "device_lists": {
    "changed": [],
    "left": []
  },
  "presence": {
    "events": []
  },
  "rooms": {
    "join": {},
    "invite": {},
    "leave": {}
  },
  "groups": {
    "join": {},
    "invite": {},
    "leave": {}
  },
  "device_one_time_keys_count": {
    "signed_curve25519": 50
  },
  "next_batch": "s1195808412_757269739_794431_REDACTED_1021253_37912961_19608850_66812"
}

According to the spec, only next_batch is required, so it could be acceptable to reduce it to:

{
  "next_batch": "s1195808412_757269739_794431_REDACTED_1021253_37912961_19608850_66812"
}

I do not know if it will solve the global climat changing problem, but if it's easy to fix, let's save some bytes!

[bmarty]

I also observe empty elements in the rooms.join part, by instance:

   "rooms": {
     "join": {
       "!gGkYcJLTzcNrfERksd:matrix.org": {
         "timeline": {
           "events": [], HERE
           "prev_batch": "s1222404111_757269739_4298397_414307685_257267759_1042954_39694860_20347120_70307",
           "limited": false
         },
         "state": { HERE
           "events": []
         },
         "account_data": { HERE
           "events": []
         },
         "ephemeral": {
           "events": [
             {
               "type": "m.receipt",
               "content": {
                 "$QIzSpVAHOcFGWWrVc6eXDc0wMBrAj87qtszSyO2TsEU": {
                   "m.read": {
                     "@kindly:naido.org": {
                       "ts": 1578909070063
                     }
                   }
                 }
               }
             }
           ]
         },
         "unread_notifications": {
           "notification_count": 27,
           "highlight_count": 0
         },
         "summary": {} HERE
       }
     }
   }

[deepbluev7]

#9919 fixes the first part, but it doesn't go into rooms.join yet.

[erikjohnston]

This breaks the current release of Element iOS fwiw, so we should wait for them to fix it and push out a release before doing this. c.f. element-hq/element-ios#4309

[richvdh]

I think this is done now?

[deepbluev7]

Was there any PR to strip empty keys from inside room.join for example?