List available frontends also when no static/frontends folder is present yet

* To see what front ends are installed, it ls static/frontends. When this folder doesn't exists yet, it will return an empty array.
* Installing still works since the folder is created during installation already

Backport of: https://git.pleroma.social/pleroma/pleroma/-/merge_requests/3510

lib/pleroma/web/admin_api/controllers/frontend_controller.ex

@@ -35,6 +35,12 @@ defmodule Pleroma.Web.AdminAPI.FrontendController do
   end
 
   defp installed do
-    File.ls!(Pleroma.Frontend.dir())
+    frontend_directory = Pleroma.Frontend.dir()
+
+    if File.exists?(frontend_directory) do
+      File.ls!(frontend_directory)
+    else
+      []
+    end
   end
 end