MDX adds trailing whitespace to empty lines when trying to preserve indentation.

[NathanReb]

MDX preserves the indentation from the first line of a cram block, e.g running mdx on the following code block:

```sh
  $ echo "Hello!"
```

results in:

```sh
  $ echo "Hello!"
  Hello!
```

Problem is, it does it for empty lines as well, adding trailing whitespaces.