How to retrieve null lookup entries on mongodb?

I have this query that provides me the join I want to:

db.summoners.aggregate([
    { "$match": { "nick":"Luispfj" } },
    { "$unwind": "$matches" },
    {
        "$lookup": {
            "from":"matches",
            "localField":"matches.gameId",
            "foreignField":"gameId",
            "as":"fullMatches"
        }
    },
    { "$unwind": "$fullMatches" },
    {
        "$group": {
            "_id": null,
            "matches": { "$push":"$fullMatches" }
        }
    }
])

But when I run the unwind function the null entries are gone. How do I retrieve them (with their respective "gameId"s, if possible?

Also, is there a way to retrieve only the matches array, instead of it being a subproperty of the "null-id-object" it creates?