Handle empty TomDoc comment as an empty string

[miyagawa]

When you call .tomdoc on a method without any TomDoc documentation, the method will throw errors:

NoMethodError: undefined method `sub' for nil:NilClass
   .../tomdoc/lib/tomdoc/tomdoc.rb:130:in `tomdoc'

You can work around by calling .valid? before calling it, but other method such as raw works fine, and the error message isn't useful anyway.

This patch fixes it by simply returning '' for comment.tomdoc in case of no comments.