Only complete write_post on Tumblr posts w/ non-nil content

Answers to questions on Tumblr appear as posts without content.
Whilst looping through the posts we are therefore to ignore all nil-content posts as ```JekyllImport::Importers::Tumblr#write_post``` fails when it tries adding ```nil``` to a string.
```ruby
f.puts post[:header].to_yaml + "---\n" + nil
```
Perhaps we **do** want to add these posts in which case we'll at least have to ensure we don't attempt to append ```nil``` to the string by perhaps opting for
```ruby
f.puts post[:header].to_yaml + "---\n" + (content or "")
```