setting theme name back to an empty string

Setting this to !!null will not work, submitted a bug against ansible so
hopefully this can be resolved.

https://github.com/ansible/ansible/issues/4812

Until then I will fix edx-platform as a work-around to not
use an empty string as an import

playbooks/roles/edxapp/defaults/main.yml

@@ -393,10 +393,10 @@ worker_core_mult:
   cms: 2
 
 # Theming
-# To turn off theming, specify edxapp_theme_name: !!null
+# To turn off theming, specify edxapp_theme_name: ""
 # Stanford, for example, uses edxapp_theme_name: 'stanford'
 #
-edxapp_theme_name: !!null
+edxapp_theme_name: ""
 edxapp_theme_source_repo: 'https://{{ COMMON_GIT_MIRROR }}/Stanford-Online/edx-theme.git'
 edxapp_theme_version: 'HEAD'