This causes a problem when you run `--skip-tags deploy`

fix mode for verified user

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

fixing default for THEME_NAME

If THEME_NAME is set to an empty string it will cause a sass error due
to this mako code:

```
% if env.get('THEME_NAME') is not None:
  // import theme's Sass overrides
  @import '${env.get('THEME_NAME')}';
% endif
```