Fix nil.map call in link_helper

@gpleiss I'm seeing:

```
/home/jd/.gem/ruby/bundler/gems/hologram-bcd45252c628/lib/hologram/doc_builder.rb:190:in `block in link_helper': undefined method `map' for nil:NilClass (NoMethodError)
```

When building our current style guide with the latest hologram master. Does this seem like an appropriate fix? I've not spent enough time working out why `:blocks` is empty in the spot, but it seems like for Trulia's use case it is happening.