Removing an avatar does not update the cached value.

Original issue: https://github.com/element-hq/element-x-ios/issues/4117

To explain the issue I'll add two sync responses, one for when the avatar is updated with a new avatar URL, and another one when the avatar is updated with a null value.

## Updated the room avatar with a new avatar
```
{
"pos": "92278017/s6020200519_757284974_6174708_3755728849_4242429717_265999676_1501755517_11151313124_0_501813",
"lists": {
"all_rooms": {
"count": 1,
"ops": [
{
"op": "SYNC",
"range": [
0,
0
],
"room_ids": [
"!yaCGbiOMYrDStgWeJr:matrix.org"
]
}
]
}
},
"rooms": {
"!yaCGbiOMYrDStgWeJr:matrix.org": {
"notification_count": 0,
"highlight_count": 0,
"avatar": "mxc://matrix.org/vIjDaVKBEbwZnqWgGCUICiFZ",
"timeline": [
{
"content": {
"info": {
"mimetype": "image/jpeg"
},
"url": "mxc://matrix.org/vIjDaVKBEbwZnqWgGCUICiFZ"
},
"origin_server_ts": 1750870056555,
"sender": "@mauro-fresh-test:matrix.org",
"state_key": "",
"type": "m.room.avatar",
"unsigned": {
"replaces_state": "$ZhOcRvDWYId3ezMib5reiI3UhIDcdElU74ke3snQF0o",
"prev_content": {
"info": {
"mimetype": "image/jpeg"
},
"url": "mxc://matrix.org/LyxCQrNKUopMetbFJnAMbuwE"
},
"prev_sender": "@mauro-fresh-test:matrix.org",
"membership": "join",
"age": 253
},
"event_id": "$h9cbzBomcAA48mip7UhKfgU7f6EXIrL0nMuYfFAfOG0"
}
],
"limited": false,
"prev_batch": "s6020200518_757284974_6174708_3755728849_4242429717_265999676_1501755517_11151313124_0_501813",
"num_live": 1
}
},
"extensions": {
"account_data": {
"global": [],
"rooms": {}
},
"receipts": {
"rooms": {}
},
"typing": {
"rooms": {}
}
}
}

```
Everything works fine, the avatar gets updated and the new avatar is correctly displayed in the `RoomInfo` and in the room list

## After an avatar has been removed
```
{
"pos": "92278017/m6020205333~36.6020205358~2.6020205361~37.6020205358~1.6020205359_757284974_6183177_m3755734258~38.3755734261~75.3755734262~39.3755734261_4242436499_265999688_1501755866_11151313705_0_501816",
"lists": {
"all_rooms": {
"count": 1,
"ops": [
{
"op": "SYNC",
"range": [
0,
0
],
"room_ids": [
"!yaCGbiOMYrDStgWeJr:matrix.org"
]
}
]
}
},
"rooms": {
"!yaCGbiOMYrDStgWeJr:matrix.org": {
"notification_count": 0,
"highlight_count": 0,
"timeline": [
{
"content": {
"url": null
},
"origin_server_ts": 1750870173350,
"sender": "@mauro-fresh-test:matrix.org",
"state_key": "",
"type": "m.room.avatar",
"unsigned": {
"replaces_state": "$h9cbzBomcAA48mip7UhKfgU7f6EXIrL0nMuYfFAfOG0",
"prev_content": {
"info": {
"mimetype": "image/jpeg"
},
"url": "mxc://matrix.org/vIjDaVKBEbwZnqWgGCUICiFZ"
},
"prev_sender": "@mauro-fresh-test:matrix.org",
"membership": "join",
"age": 325
},
"event_id": "$VzfssgMRWyDhHcPqcLiSr3_9GjZIF8Cpv9CF0hztmzU"
}
],
"limited": false,
"prev_batch": "s6020205358_757284974_6183177_m3755734258~38.3755734261~75.3755734262~39.3755734261_4242436499_265999688_1501755866_11151313705_0_501816",
"num_live": 1
}
},
"extensions": {
"account_data": {
"global": [],
"rooms": {}
},
"receipts": {
"rooms": {}
},
"typing": {
"rooms": {}
}
}
}
```
What happens in this case is that the cache still keeps the existing avatar, my feeling is that the SDK is not able to differentiate between a null value, and no value present at all, which makes the cache ignore the avatar update and removing it.
Clearing the cache solves the issue and restores the empty avatar in fact.

Start by reproducing the issue with the two provided sync responses and trace how the SDK handles the room avatar when the event content contains a null URL. Verify that the cache removes the previous avatar without requiring a cache clear, and confirm that the empty avatar is then shown in the room list and RoomInfo.

Written by the indexing model from the issue text.