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
```
updating comment
Merge pull request #422 from edx/jarv/fix-theme-name

fixing default for THEME_NAME