Only compress and upload to S3 if there are CSS or JS files to compress and upload

For `simplestatic` blocks that only contain JS or CSS, a 0 byte file was being created and uploaded to S3 on `static_sync`.

For example, with the following `simplestatic` block, a 0 byte CSS file was also uploaded (but not rendered in the HTML).

```
{% simplestatic %}
    {% compress_js "js/plugins.js" %}
    {% compress_js "js/main.js" %}
    {% compress_js "js/retina.js" %}
{% endsimplestatic %}
```