Fixes spec failures from File.join being called on a blank options[:template_directory]

Current test failures:

```
.................................................................................FF.FF....FF...........

Failures:

  1) Fdoc::EndpointPresenter#to_html should generate valid HTML
     Failure/Error: html = subject.to_html
     TypeError:
       no implicit conversion of nil into String
     # ./lib/fdoc/presenters/base_presenter.rb:18:in `join'
     # ./lib/fdoc/presenters/base_presenter.rb:18:in `render_erb'
     # ./lib/fdoc/presenters/endpoint_presenter.rb:12:in `to_html'
     # ./spec/fdoc/presenters/endpoint_presenter_spec.rb:14:in `block (3 levels) in <top (required)>'

  2) Fdoc::EndpointPresenter#to_markdown should generate markdown
     Failure/Error: markdown = subject.to_markdown
     TypeError:
       no implicit conversion of nil into String
     # ./lib/fdoc/presenters/base_presenter.rb:18:in `join'
     # ./lib/fdoc/presenters/base_presenter.rb:18:in `render_erb'
     # ./lib/fdoc/presenters/endpoint_presenter.rb:16:in `to_markdown'
     # ./spec/fdoc/presenters/endpoint_presenter_spec.rb:24:in `block (3 levels) in <top (required)>'

...
```

This is caused by:

```ruby
class Fdoc::BasePresenter
  # ...
  def render_erb(erb_name, binding = get_binding)
    template_path = File.join(options[:template_directory], erb_name)
```

... Where `options[:template_directory]` is nil (ie. the default).