Fix migration for empty db.

priv/repo/migrations/20171109114020_fill_actor_field.exs

@@ -5,19 +5,20 @@ defmodule Pleroma.Repo.Migrations.FillActorField do
 
   def up do
     max = Repo.aggregate(Activity, :max, :id)
+    if max do
       IO.puts("#{max} activities")
       chunks = 0..(round(max / 10_000))
 
       Enum.each(chunks, fn (i) ->
         min = i * 10_000
         max = min + 10_000
-      IO.puts("Updating #{min}")
         execute("""
         update activities set actor = data->>'actor' where id > #{min} and id <= #{max};
         """)
         |> IO.inspect
       end)
     end
+  end
 
   def down do
   end