Fix listing of incomplete notifications

When empty string is given, then notification is displayed without proper message. Issue lies in handling such notifications in list_notifications.

For example this:

$ create_notification -s news ""
$ list_notifications
{
    "notifications": [
        {
            "displayed": false,
            "id": "1584457319-28027",
            "severity": "news",
            "messages": {
            
            }
        },
    ]
}

Non-empty string will create proper notification message

create_notification -s news "lorem ipsum"
$ list_notifications
{
    "notifications": [
        {
            "displayed": false,
            "id": "1584468192-12072",
            "severity": "news",
            "messages": {
                "en": "lorem ipsum"
            }
        },
    ]
}