Fix for String#undent on unindented strings

Without it, String#undent would fail on unindented strings, e.g.:

```rb
"foo".undent
```

gives: ``NoMethodError: undefined method `length' for nil:NilClass` ``. I added a test that fails without the fix but pass with it.


By the way, you have a pretty good contributing guide for formulae contributors, but I couldn’t find any docs regarding code contributions.