Fixed assert for Markdown.to_html("")

Ran into an assertion when trying to generate docs for this module:
```elixir
defmodule Bob do
  @doc """
  """
  def example(), do: true
end
```
```
[jw-macbook-2 /tmp/bob]$ mix docs                                                                                                                               [] 
Compiled lib/bob.ex
Generated bob.app
Assertion failed: (buf && buf->unit), function bufput, file src/buffer.c, line 157.
[1]    3912 abort      mix docs
```

Figured fixing it in the nif was the best place to catch all possible use cases. I haven't done much nif coding, so making an size 0 `ErlNifBinary` and not actually copying anything to it might not be the smartest solution; open to cleaner suggestions.