Fix exception on bad directive '@'

When sass encounters a sass-file that has a single @ character, it trips up the parser:
```
undefined method `gsub' for nil:NilClass
sass-3.2.13/lib/sass/engine.rb:716:in `parse_directive'
sass-3.2.13/lib/sass/engine.rb:566:in `parse_line'
sass-3.2.13/lib/sass/engine.rb:471:in `build_tree'
sass-3.2.13/lib/sass/engine.rb:490:in `block in append_children'
sass-3.2.13/lib/sass/engine.rb:489:in `each'
sass-3.2.13/lib/sass/engine.rb:489:in `append_children'
sass-3.2.13/lib/sass/engine.rb:345:in `_to_tree'
sass-3.2.13/lib/sass/engine.rb:274:in `to_tree'
```

In this case `directive` is `nil` so it triggers the else branch, which tries to call gsub on it.