Can't add custom children on remote ast

[wilkerlucio]

I found one issue with the current implementation for the ast->expr regarding mutation joins. Given this case:

(def parser (om/parser {:mutate (fn [{:keys [ast]} _ _]
                                  {:remote (assoc ast :children (:children (om/query->ast [:id])))})}))

(parser {} [`(do-something {:foo "bar"})] :remote)
; expected => `[{(do-something {:foo "bar"}) [:id]}]
; current  => `[{(do-something {:foo "bar"}) nil}]

The children are not getting to the final query output.