Add EnsureUserKeyPlug, smaller fixes

lib/pleroma/plugs/authentication_plug.ex

@@ -37,4 +37,6 @@ defmodule Pleroma.Plugs.AuthenticationPlug do
     Pbkdf2.dummy_checkpw()
     conn
   end
+
+  def call(conn, _), do: conn
 end

lib/pleroma/plugs/ensure_user_key_plug.ex

@@ -0,0 +1,14 @@
+defmodule Pleroma.Plugs.EnsureUserKeyPlug do
+  import Plug.Conn
+
+  def init(opts) do
+    opts
+  end
+
+  def call(%{assigns: %{user: _}} = conn, _), do: conn
+
+  def call(conn, _) do
+    conn
+    |> assign(:user, nil)
+  end
+end