Notifications endpoint returns invalid account attribute for notification item when user has been deleted

api/v1/notifications returns invalid account attribute for notification item when the original user has been deleted from their corresponding instance since the notification was created.

Steps to replicate:

1. Create User A on Instance 1, and User B on Instance 2.
2. Send a direct message to User B from User A.
3. As admin of Instance 1, delete User A.
4. Fetch User B's notifications using api/v1/notifications endpoint, observe empty accounts object in JSON response.

Mastodon API indicates that the accounts object should not be null, so clients will not be expecting a null response (e.g. crash observed in Roma).

Sample response:

[
    {
        "account": {},
        "created_at": "2019-07-12T15:14:15.000Z",
        "id": "304803",
        "pleroma": {
            "is_seen": false
        },

        ...

        "type": "mention"
    }
]