Remove empty nested rulesets (#133)

1. Fix for rulesets with spaces only: `a {      }` is now treated as an empty ruleset.
2. Fix for preprocessors: nested rulesets are removed if they are empty.

Ruleset is now considered empty if it has nothing but spaces or another empty ruleset.
Example:

```scss
// Before:
.parent1 {
    .child {
        .grandchild { }
    }
}
.parent2 {
    .child1 {
        @include 'panda';
    }
    .child 2 {
        .grandchild { }
    }
}

// After:
.parent2 {
    .child1 {
        @include 'panda';
    }
}
```
Merge pull request #139 from csscomb/tg/133-empty-rulesets

Remove empty nested rulesets (#133)