p/lift-placeholders to "walk" into query params

[souenzzo]

Minimal reproduce

(map? (p/lift-placeholders-ast
        {}
        {:type :root 
         :children [{:type :prop 
                     :dispatch-key :a 
                     :key :a 
                     :params {:a (reify clojure.lang.IPersistentMap)}}]}))
Execution error (AbstractMethodError) at user$eval22722$reify__22723/empty (form-init7704631119162152748.clj:-1).
Method user$eval22722$reify__22723.empty()Lclojure/lang/IPersistentCollection; is abstract

I found this issue when trying to use a datomic db as a pc/input with pc/reader3