Bugfixes for empty require configurations

Hi there,

thank you for this module! I had a usage case where the config file looks like this:

```javascript
requirejs.config({

});
```
it stopped with a unreadable error: `TypeError: Cannot read property '1' of null`. Its becase the indentation regexp does not match.
So I wanted to create some tests for it and fix it
I put a new describe - area to the current tests and had to hack the others a little bit to use another (empty) fixture. I hope you don't mind it. If you have another / cleaner idea, please let me know. This is my first contribution with mocha.
I wrote two tests, because I couldn't get the TypeError with just one test.

After all I fixed the bugs. The default for files without indentation is now `"  "` I could change that if you want to.

Best regards
Philipp

P.S.: hope you don't mind a gitignore