Edit fallback fails when edit event itself is broken

Relates to https://github.com/matrix-org/matrix-appservice-slack/issues/322 much, where currently appservice-slack doesn't generate proper edit event towards matrix when slacker has edited an message. While that is ofcourse bug on it's own, riotX fails to show the fallback edit nor the "real" edit, resulting that user on riotX will not see any edit being done, when there is one done...

Screenshots:
riotX (see no edit nor fallback seen)
riot-android (see only fallback as nothign else is supported)
riot-web (shows fallback because actual edit event is broken, this is how riotX should show it too)

Message sources, Original message:
```
{
"type": "m.room.message",
"sender": "@slack_helsinkihacklab_:hacklab.fi",
"content": {
"body": "Turvaominaisuus, ei saa ainakaan molempia käsiä liikkuvien osien cäliin!",
"format": "org.matrix.custom.html",
"formatted_body": "Turvaominaisuus, ei saa ainakaan molempia käsiä liikkuvien osien cäliin!",
"msgtype": "m.text"
},
"event_id": "$15737336231061FYGFD:hacklab.fi",
"origin_server_ts": 1573733623423,
"unsigned": {
"age": 1125
},
"room_id": "!MjVdMmsFAxVzHUZLiV:matrix.org"
}
```

And from the edit:
```
{
"type": "m.room.message",
"sender": "@slack_helsinkihacklab_:hacklab.fi",
"content": {
"body": "(edited) ... osien cäliin! => ... osien väliin! ",
"format": "org.matrix.custom.html",
"formatted_body": "(edited) ... osien cäliin! => ... osien väliin! ",
"m.new_content": {
"body": "Turvaominaisuus, ei saa ainakaan molempia käsiä liikkuvien osien väliin!",
"format": "org.matrix.custom.html",
"formatted_body": "Turvaominaisuus, ei saa ainakaan molempia käsiä liikkuvien osien väliin!",
"msgtype": "m.text"
},
"m.relates_to": {
"rel_type": "m.replace"
},
"msgtype": "m.text"
},
"event_id": "$15737336241062ZmRbl:hacklab.fi",
"origin_server_ts": 1573733624638,
"unsigned": {
"age": 172
},
"room_id": "!MjVdMmsFAxVzHUZLiV:matrix.org"
}
```