fix(rpc): handle nil value in preapply result

Author: Awea

lib/rpc.ex

@@ -239,7 +239,7 @@ defmodule Tezex.Rpc do
 
   defp do_preapply_operation(%Rpc{} = rpc, payload) do
     case post(rpc, "/blocks/head/helpers/preapply/operations", payload) do
-      {:ok, [%{"contents" => preapplied_operations}]} ->
+      {:ok, [%{"contents" => preapplied_operations}]} when is_list(preapplied_operations) ->
         applied? =
           Enum.all?(
             preapplied_operations,