Fix for empty code fences

When there's an empty code fence, polytexnic breaks:

```
/usr/local/var/rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/polytexnic-0.9.3/lib/polytexnic/utils.rb:209:in `add_font_info': undefined method `gsub!' for nil:NilClass (NoMethodError)
	from /usr/local/var/rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/polytexnic-0.9.3/lib/polytexnic/utils.rb:141:in `block (2 levels) in highlight_source_code'
...
```

To replicate, I'm just doing something like:

```ruby
```
Merge pull request #102 from benjamintanweihao/patch-1

Fix for empty code fences
Add a failing test for broken table

[#70476292]
Fix tables with center env

Work by reducing to a previously solved problem (for figures).

[Delivers #70476292]
[#70476292]

* 70476292-table-xrefs:
  Fix tables with center env
  Add a failing test for broken table
Add a kludge for footnotesize in tables

[Delivers #70476292]
[#70476292]

* 70476292-table-xrefs:
  Add a kludge for footnotesize in tables
Merge branch 'master' into stable