fixup! ✨(edxapp) add support for LMS themes

Author: jmaupetit

apps/edxapp/vars/all/main.yml

@@ -13,8 +13,8 @@ edxapp_django_port: 8000
 #
 #   edxapp_theme_url: "https://github.com/raccoongang/themes_for_themex.io)"
 #   edxapp_theme_tag: "marvel-theme-ginkgo"
-edxapp_theme_url: null
-edxapp_theme_tag: null
+edxapp_theme_url: ""
+edxapp_theme_tag: ""
 edxapp_build_settings: "fun.docker_build_production"
 
 # -- memcached