Config.get/2 behaves differently for `nil` values

if list is given as key and actual config value is nil, then default value will be returned.

if an atom is given as key and actual config value is nil, then nil will be returned.

config :pleroma,
  wakawaka: nil

config :pleroma,
  makamaka: %{lorem: nil}

iex(master_daadc62f@127.0.0.1)1> Pleroma.Config.get(:wakawaka, :wut)
nil
iex(master_daadc62f@127.0.0.1)2> Pleroma.Config.get([:makamaka, :lorem], :wut)
:wut

We should probably unify the behaviour, any objections?